Melt butter in a large saucepan, then add coffee and 5 tablespoons of cocoa. Once the mixture comes to a boil, move it off the heat and gently stir in dry ingredients.
Fold in sour cream, then add eggs and vanilla, stirring until combined.
Pour batter into a lightly greased 10”x 15” pan and bake 18-22 minutes. When the cake is done, a toothpick inserted into the center will come out clean.
Place cake on a cooling rack and prepare glaze.
Glaze
Melt 1/2 cup butter in a large saucepan. Stir in milk and cocoa. Bring mixture to a boil, then remove from the stovetop.
Using a hand-mixer or whisk, add powdered sugar and mix until smooth. Stir in vanilla, then pour over the warm cake.