Pinwheel Cookies

INGREDIENTS:
1/2 cup butter (soft, at room temperature)
1/2 cup white granulated sugar
1 egg yolk
2 teaspoons vanilla
A pinch of salt
1/2 tablespoon baking powder
1 tablespoon milk
1 1/2 cups all-purpose flour
1 tablespoon butter
1 tablespoon milk
3 tablespoons unsweetened cocoa powder