Preheat oven to 325 degrees F (165 degrees C). Grease a 9x13-inch baking pan; dust pan with 2 teaspoons cocoa powder or as needed.
Whisk cake flour, 3/4 cup cocoa, baking soda, baking powder, and salt in a bowl. Mix sugar and canola oil in a mixing bowl and beat with an electric mixer on medium speed until combined. Beat eggs into sugar and oil, one at a time, blending in the first egg before adding the second. Gradually beat milk, coffee, and vanilla extract into sugar mixture until smooth.
Reduce mixer speed to low and beat flour mixture into wet ingredients. Stir sour cream into batter. Pour into prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean or with moist crumbs, about 40 minutes.