Preheat the oven to 350 degrees F. Lightly oil and dust with flour either one (9- by 1 1/2-inch) round cake pan or two (9-inch) cake loaf pans.
In a large bowl, sift together the flour, baking powder, and salt; set aside.
In a standing mixer fitted with the paddle attachment or with a hand-held electric mixer in a large bowl, mix together (over low speed) the sugar and butter. Gradually mix in the eggs, one at a time, until the mixture turns a light yellow. Mix in the vanilla extract.
Turning the speed up to medium, mix in 1/2 cup of the milk and 1/2 of the flour mixture until combined. Stop and scrape the bowl down, then add remaining 1/2 cup of milk and remaining flour mixture; mix for two additional minutes until batter is smooth and creamy; set aside.
In a medium-sized bowl, whisk the cocoa powder with 1/4 cup of very hot water until smooth. Add in approximately 1 cup of the prepared cake batter and mix together until fully combined and smooth. This will be your chocolate marbling cake batter.
Marbling the cake: Pour about 1/2 of the yellow cake batter into the bottom of the cake pan. Drop large spoonfuls of the chocolate cake batter as the next layer. Pour the remaining yellow cake batter as last layer on top.
To create the marble effect, insert a spoon or butter knife into cake batter. Move the utensil in large swirling motions to see yellow and chocolate cake batter take on a marbled appearance. Be careful not to over swirl! You do not want to blend colors together.
Bake approximately 30 to 40 minutes, rotating the pan/pans halfway through the cooking (once you reach 30 minutes, check cake every few minutes until done). Cake is done when a toothpick inserted in the center of the cakes comes out clean or when the internal temperature registers approximately 205 to 209 degrees F. on your cooking thermometer.
Remove the cakes from the oven and run a knife around the edges to loosen them from the sides of the pans. Invert cake onto a plate and then re-invert onto a cooling rack, rounded-sides up. Let cool completely before frosting. While the cake is cooling make the Chocolate Mocha Frosting. Once the cake is cooled, frost the cake with the frosting. The cake can be stored at room temperature for 3 to 4 days. Wrap in plastic wrap or foil to seal in moisture or cover with airtight lid.
To freeze: Double wrap in foil and slice off a serving when you have a craving for cake!
Makes a cake large enough to serve 6 to 8 people (depending on the size of the slices).
In small bowl, sift together the powdered sugar, cocoa powder and instant coffee; set aside.In a standing mixer (fitted with the paddle attachment) or with a hand-held electric mixer in a large bowl, mix the butter and vanilla on low speed until blended. Increase the speed to medium, and gradually mix in the powdered sugar mixture. If mixture starts to get crumbly, start adding the milk, one tablespoon at a time, until the frosting is smooth and has a fluffy texture. If you want a thinner frosting add a little more milk. Occasionally turn the mixer off, and scrape the down the sides of the bowl with a rubber spatula.
NOTE: Frosting may be stored, covered, in the refrigerator for up to 3 days.
Using a palette knife or spatula, spread some of the chocolate mocha frosting over the top and sides of the cake (spread enough frosting to make a 1/4- to 1/2-inch layer of frosting). Sprinkle the top with various colored sprinkles, if desired.Makes enough to frost a 1 layer (9- x 1 1/2-inch round) cake or 2 (9-inch) loaf cakes. Double the frosting recipe if you wish to make a 2 layer cake.
German Marble Cake History and Recipe: https://whatscookingamerica.net/History/Cakes/GermanMarble-Cake.htm