Preheat Oven & Prep Pan:Preheat your oven to 400°F (205°C) and grease a sheet pan.
Mix Dry Ingredients: In a large bowl, mix together the flour, white sugar, and cocoa powder.
Prepare Wet Ingredients: In a small saucepan, add the water, butter, shortening, and German baking chocolate. Bring the mixture to a boil over medium heat. Once it is smooth and fully combined, pour it into the bowl with the dry ingredients and whisk until combined.
Finish the Batter: Stirring between each addition, add the buttermilk, baking soda, salt, eggs, and vanilla. Mix until the batter is smooth.
Bake the Cake: Pour the batter into the prepared sheet pan and spread it evenly. Bake for 20-22 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely before frosting.