This easy Texas Sheet Cake will become your go-to treat for special occasions and everyday indulgences alike. Buttermilk keeps the cake moist and a creamy fudgy frosting adds the perfect finishing touch.
Sift the flour and sugar together in a mixing bowl. Set aside.
In a small saucepan, combine the oil, butter, cocoa powder, and water. Bring the mixture to a boil, stirring until smooth.
Add the butter mixture to the flour and sugar and stir until incorporated.
Add buttermilk, eggs, vanilla, baking soda, and salt and mix until fully combined.
Pour into ungreased half-sheet pan (18 x 13 in) (Note 2) and bake 15-18 minutes or until a cake tester inserted in the center comes out clean. Cool for 3-5 minutes before frosting.
Sift the powdered sugar to the bowl of a stand mixer fitted with a whisk attachment.
In a saucepan, combine butter and cocoa powder. Bring to a boil, stirring until smooth.
Add the butter mixture to the powdered sugar along with the vanilla extract. Whisk until well incorporated. The mixture will be thick and dry.
Add 6 tablespoons of buttermilk and blend - starting slowly - until combined. Continue whisking until smooth and creamy. Add the remaining buttermilk as needed to create a spreadable consistency.
Spread the frosting on the warm cake using an offset spatula. When completely cool, cover with plastic wrap and store at room temperature for 3-4 days.
Gather and measure the ingredients ahead of time. This cake comes together quickly so it's most helpful to have the ingredients ready and within easy reach.
The pan can be lined with parchment paper, but this is not necessary.
For small-batch baking, change the number of servings in the recipe card to 12. The ingredient amounts will recalculate. Prepare as noted above. Bake in a quarter sheet pan for 9-11 minutes or until a cake tester inserted in the center comes out clean.
Sift the powdered sugar to eliminate lumps in the frosting.
To freeze the cake, bake, frost, and cool the cake completely. Flash freeze until firm. Then cover with plastic wrap, then aluminum foil and freeze for up to 2 months. Thaw and serve.