To prep the pan:

1. Preheat the oven to 350 degrees F.
2. Butter the sides and bottom of a half sheet pan, and line the bottom with parchment paper. The butter on the bottom of the sheet will help the parchment stay in place.
3. Use a food safe marker to draw a line down the center of the parchment, between the long sides of the baking sheet. You should have two equal halves that are almost perfect squares. This makes it easier to see where the two batters should meet.
4. Set the prepared pan aside.

Melt the chocolate and prep the dry ingredients:

1. Add the 3 oz of chopped chocolate to a small, heatproof bowl and microwave on full power for 30 seconds. Carefully remove the bowl from the microwave and stir the chocolate to distribute the heat. Microwave again for another 30 seconds and stir. Finally, heat one more time for 20-25 seconds and stir until melted completely. 
2. If you need to heat the chocolate again, only do it for 5 second intervals, stirring in between. Set the melted chocolate aside until you need it at the very end.
3. For the chocolate batter, sift together the flour, cocoa powder, baking powder, salt and espresso powder into a medium sized mixing bowl. Whisk the sifted dry ingredients and set the bowl aside.
4. For the vanilla batter, sift together the flour, baking powder and salt into another medium sized mixing bowl. Whisk the sifted dry ingredients and set the bowl aside.

Make the batter for each cake flavor:

1. You will need 2 large mixing bowls and an electric hand mixer. Add 1/4 cup butter to each mixing bowl, followed by 1 cup of sugar. Beat the butter and sugar in the first bowl for 1 1/2 – 2 minutes, or until the mixture looks like wet sand. Repeat for the other bowl.
2. Add 1/3 cup of vegetable oil to the bowl (it doesn’t matter which one) and beat for another minute on medium high speed. The mixture should be nearly white and fluffier. Repeat for the other bowl.
3. Add one egg to the bowl (it doesn’t matter which one) and beat on medium speed until well blended. Add the second egg to the same bowl and mix until incorporated. Repeat with the other bowl.
4. Decide which bowl will have the vanilla batter, and add 1/2 cup sour cream along with 1 1/2 tsp of vanilla extract. Use a sticky note or some colorful tape to keep the bowls clearly marked. Beat in the sour cream and vanilla for about 10 seconds until well blended.
5. Repeat step 4 with the chocolate batter, adding 1/2 cup sour cream and only 1/2 tsp vanilla extract. Beat until well blended.
6. Finish the vanilla cake batter first, so that you won’t need to wash the beaters before jumping right in to the chocolate batter. Add 1/2 of the dry ingredients (without the cocoa powder) and mix on medium low speed until incorporated.
7. Stream 1/2 cup of whole milk into the vanilla batter and mix until incorporated. Follow with the remaining dry ingredients (without the cocoa powder) and decrease the speed to low until the flour is mostly mixed in. Stop the mixer and use a spatula to fold the vanilla batter a few times and gently finish mixing.
8. Set the vanilla batter aside while you move on to finish the chocolate. Add half of the dry ingredients (with the cocoa powder) to the chocolate batter bowl and mix on medium low speed until well blended.
9. Stream in 1/2 cup whole milk and mix on medium low speed until well blended. Follow with the remaining dry ingredients (with the cocoa powder) and the melted chocolate. Decrease the mixer speed to low and mix until the flour is mostly mixed in. Shut off the mixer and use a spatula to fold the chocolate batter a few times and gently finish mixing.

Bake the cake:

1. Scrape the chocolate batter onto one side of the sheet pan and the vanilla onto the opposite side, using the line you drew before as a reference. The batter is thicker and doesn’t run easily, so you’ll have time to gently spread each flavor into place and smooth the surface.
2. Carefully load the sheet pan into the oven and bake for 20-22 minutes, rotating the pan midway through baking. You can skip rotating the pan if your oven bakes evenly, but if your oven has hot spots, I highly recommend turning the cake for a better result.
3. The cake is done if it springs back slowly when pressed with your finger. Carefully remove the cake from the oven and allow it to cool completely before frosting.

Make the vanilla frosting:

1. Combine the 1/2 cup butter, 2 cups powdered sugar, 1/2 tsp salt, 2 tsp vanilla extract, and 2 TB heavy cream in a large mixing bowl. Using an electric hand mixer and beaters, beat the mixture on medium low speed until the butter has mostly soaked up the dry sugar, then increase the speed to medium and continue mixing for about 15 seconds. The frosting should be smooth, don’t overbeat, or you’ll get a lot of air bubbles.
2. Use the vanilla frosting to evenly cover one long half of the sheet cake, making a “cross” with the center line that was made by the cake flavors meeting together. The frosting half should be perpendicular to the cake half. See visual aids in post for more help.
3. Scrape out the bowl and beater(s) well, and use them to make the chocolate frosting. No need to wash dishes at this point!

Make the chocolate frosting and finish the cake:

1. In the bowl that you used for the vanilla frosting, combine 1/2 cup butter, 2 cups powdered sugar, 2/3 cup cocoa powder, 1/4 tsp salt, 1/3 cup heavy cream and 2 tsp vanilla extract. Beat on medium low speed until the butter has soaked up the sugar and cocoa, then increase the speed to medium and beat for about 30 seconds. The mixture will lighten a bit and become creamy.
2. Spread the chocolate frosting onto the other long half of the cake, right beside the vanilla. Decorate with sprinkles as desired and serve! Cake can also be chilled to harden the chocolate frosting before serving, it’s your choice.
3. Leftover iced cake will keep in the fridge for about a week. Freeze indefinitely.