The yellowcake is moist and light. The use of cake flour helps the cake from drying out and keeps it fresh for a few days. The chocolate frosting combines milk chocolate and cocoa, developing a rich deep flavor.
Preheat oven to 350 degrees F
Lightly spray and flour a 9X13 baking pan
In a mixing bowl, whisk flour, baking powder, salt and baking soda.
In the bowl of a stand mixer beat the butter and oil on medium speed until smooth.
Beat in the sugar on medium until fluffy, 3 minutes.
Add the eggs and 3 yolks, one at a time, beating after each addition.
Beat in the vanilla and the remaining egg yolk.
Add ⅓ of the flour and mix just until blended.
Add half of the buttermilk and blend.
Add another ⅓ of the flour and beat on low until blended.
And then beat in the remaining flour.
Pour the batter evenly into the 9x13 pan.
In a bowl, of a stand mixer to beat the butter and cream cheese at medium speed until creamy.
add the powdered sugar and cocoa powder, beat on low speed until blended.
Increase speed to medium, and gradually add in milk and vanilla, beat until smooth.
Microwave the chocolate on medium for 30 second intervals until smooth, stirring at 30 second intervals.
Gradually add melted chocolate to the mixture; beat until smooth.
Frost the cake using an offset spatula covering the entire cake.