PREHEAT OVEN AND PREP PAN: Preheat your oven to 350°F. Grease and flour a 9 by 13 inch baking pan. Set aside.
MIX DRY INGREDIENTS: In a large bowl, whisk together 1 3/4 cups granulated sugar, 2 cups all purpose flour, 6 tablespoons Dutch processed cocoa powder, 1 1/2 teaspoons baking powder, and 1/2 teaspoon kosher salt. Break up every clump. The mixture should look even and fluffy.
HEAT COLA MIXTURE: In a small saucepan, melt 1/2 cup (1 stick) unsalted butter with 1 cup Coca Cola and 1/2 cup vegetable oil over medium heat. Warm until the butter melts and the surface shows soft bubbling around the edges. You will smell warm cola and chocolate. Remove from heat.
COMBINE WET AND DRY: Pour the hot cola mixture into the dry ingredients. Using an electric hand mixer with the beaters attachment, mix on medium speed for about 1 minute. The batter will loosen quickly, turn glossy, and have a thin pudding-like texture.
ADD REMAINING WET INGREDIENTS: Add 2 large eggs, 1/2 cup buttermilk, and 1 teaspoon pure vanilla extract. Mix on medium speed for another minute. The batter should look smooth and pourable with a deep chocolate aroma.
BAKE: Pour the batter into the prepared pan. Bake for 30 minutes, or until the top springs back lightly when touched and a toothpick inserted into the center comes out with moist, cooked crumbs.
START FROSTING IMMEDIATELY: When the cake comes out of the oven, begin the frosting. In a medium saucepan, melt 1/2 cup unsalted butter with 3 tablespoons unsweetened cocoa powder, 6 tablespoons whole milk, and 1/4 teaspoon kosher salt. Heat until the mixture just begins to bubble and you see light steam. Remove from heat.
FINISH FROSTING: Add 4 cups powdered sugar and 1 teaspoon pure vanilla extract. Whisk until smooth, glossy, and pourable. You should not see any dry streaks of powdered sugar.
FROST THE WARM CAKE: Pour the warm frosting evenly over the hot cake. It will melt slightly into the top, forming a soft fudgy layer. Let the cake cool completely so the frosting can set.
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Notes
Make Ahead: Bake and frost up to 1 day ahead. Keep covered at room temperature.Storage: Cover leftover cake and store at room temperature for 3 days, or refrigerate up to 5 days. Bring to room temperature before serving.Freezing: Freeze frosted or unfrosted slices for up to 2 months. Wrap each slice tightly in plastic wrap, then place in a freezer safe bag. Thaw at room temperature.Reheating: Warm individual slices in the microwave for 10 to 15 seconds to soften the frosting and bring back that fresh baked texture.Ingredient Tip: Dutch processed cocoa makes a deeper, smoother chocolate flavor. Regular unsweetened cocoa works, but the flavor will be lighter and a bit sweeter.