In a separate bowl, beat zest, 1/2 cup butter, 1 cup sugar and 1 tsp. vanilla on medium high speed with electric mixer until pale and fluffy, about 4 minutes. Set aside.
Add 1/2 cup of butter to a 9 inch round cake pan, and place in oven to melt.Add 1 teaspoon vanilla and 1/2 cup of sugar to melted butter in cake pan and whisk to combine.