Preheat oven to 350°F. Spray a 10x15 sheet cake pan with cooking spray (I like the kind with flour).

Whisk flour, sugar, cocoa, baking soda, salt, and coffee granules in a large bowl. Set aside.

Whisk oil, cola, buttermilk, and vanilla in a measuring cup. Stir wet ingredients into dry and stir until just blended and smooth.

Pour batter into prepared pan. Bake for about 15-22 minutes until a toothpick comes out clean 2” from the edge of the pan. The cake will somewhat spring back when touched.

As soon as the cake comes out of the oven, make the frosting. Heat the butter, cocoa, cola, and coffee granules in a small saucepan until it boils and the butter is melted. Remove from heat and stir in vanilla, salt, and powdered sugar. Pour evenly and carefully over cake, spreading as needed. Sprinkle with chopped nuts.

Cool completely before serving. Store loosely covered on counter or in refrigerator for up to 3 days.