Preheat oven to 350°. Spray a 13x9-inch pan with baking spray with flour.
For cake:
Prepare cake mix according to package directions, substituting buttermilk for water and butter for oil, and adding egg whites.
Bake until a wooden pick inserted in center comes out clean, about 30 minutes. Let cool completely.
For frosting:
In a large bowl, beat butter with a mixer at medium speed until smooth. Whisk together confectioners’ sugar and cocoa. Gradually add sugar mixture, milk, and vanilla to butter, beating 2 to 3 minutes until fluffy and light in color. Spread frosting on top of cake. Garnish with candy eggs, desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/easter-candy-sheet-cake/