Measure the flour and crack the eggs. While the butter and sugar are creaming, measure out the flour, baking powder, and salt into a small bowl and whisk lightly to combine. Crack the eggs into a bowl and add the vanilla.
Add the flour and buttermilk, alternating. Add 1/3 of the flour to the batter and beat on low speed just until incorporated. Then add 1/2 of the buttermilk and beat. Add another 1/3 of the flour, and beat, and then the last of the buttermilk and beat. Finally, beat in the last 1/3 of the flour.
Add the chocolate. Transfer roughly 1/2 of the batter to a separate bowl. In a small bowl or ramekin, mix the cocoa powder with 2 tablespoons of hot tap water until smooth. Add this chocolate mixture to the remaining batter. Beat to incorporate.
Spoon into the prepared pan, creating a two-layer checkerboard. Beginning with the vanilla mixture, place four spoonfuls of batter into the pan, spacing them far enough apart so that you can create a checkerboard when you add the chocolate batter.
Bake the cake. Place in the oven and bake for about 40 minutes, rotating the pan half way through. The cake is done when it has browned slightly and a cake tester comes our clean when inserted into the middle.
Make the ganache.
Bring the cream to a simmer in a small saucepan. Remove from heat, and add chopped chocolate and corn syrup (if using). Let sit until chocolate is melted and then stir gently to combine. Let cool slightly until ganache has thickened to a pourable consistency.
The corn syrup is optional in the ganache, but it does help with the silky texture. You can purchase corn syrup that does not contain high-fructous corn syrup at places like Whole Foods and natural food stores.
For the ganache frosting: