Grease a 13x9-inch baking dish with cooking spray. Preheat oven to 350°F.
Sift together the flour, cocoa powder, salt, and baking soda, and cinnamon. (Or at the very least, sift the cocoa powder, as it is prone to causing lumps in cake batter.)
Pour the melted butter into a large bowl. Whisk in the Dr. Pepper and the sugar until smooth. Whisk in the eggs, buttermilk, and vanilla.
Whisk in the dry ingredients until a smooth cake batter forms.
Pour into the prepared baking dish and bake for 32–38 minutes, or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the icing: Place the butter and Dr. Pepper in a saucepan. Heat until the butter is melted, but do not boil. Remove from the heat and sift in the cocoa powder (prevents lumps). Add the remaining ingredients. Mix well.
Pour the icing over the hot cake, right after it comes out of the oven. Cool completely and cut into squares.
Notes
Don’t have buttermilk? Measure 2 teaspoons of vinegar OR lemon juice into a measuring cup, then fill to the ½ cup line with regular milk.Storage and freezing: store the cake at room temperature or in the refrigerator for 4–5 days, tightly covered. It can also be frozen for up to 3 months. Defrost at room temperature. Variations:
Makes 24 cupcakes, bake 15–18 minutes
Makes one bundt cake, bake 60 minutes. Let cool for 10 minutes before inverting and frosting.