White Texas Sheet Cake

3 egg whites

1 (15.25-ounce) box white cake mix (I used Duncan Hines)

1/2 cup salted butter, melted

3/4 cup milk 

1 teaspoon vanilla extract

Icing:

1/2 cup butter

1/2 cup milk or buttermilk

1 teaspoon vanilla extract

1 (16-ounce) box powdered confectioners sugar (4 cups)

1 cup roasted pecan pieces or coarsely chopped roasted pecans