Preheat the oven to 350°F (175°C).
Grease a 9×13 inch sheet pan with butter or non-stick spray.
In a large mixing bowl, combine the boxed yellow cake mix, 1 cup water, 1/3 cup vegetable oil, and 3 large eggs.
Beat the mixture with an electric mixer on medium speed for 2 minutes until smooth.
Pour the batter evenly into the prepared sheet pan.
Bake in the preheated oven for 28 to 32 minutes or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack.
While the cake cools, prepare the frosting: In a large bowl, beat 1 cup softened unsalted butter until creamy.
Gradually add 4 cups powdered sugar and 1/4 cup unsweetened cocoa powder, mixing on low speed.
Add 1/4 cup milk and 1 teaspoon vanilla extract, then beat on high speed for 3 minutes until light and fluffy.
Once the cake is completely cooled, spread the frosting evenly over the top using a spatula.
Cut the sheet cake into 12 equal squares and serve.