Using a 2 3/4" round cutter, cut 36 circles out of piecrusts. Place in ungreased miniature muffin pans, crimping edges.
Beat butter and brown sugar in a large bowl at medium speed with an electric mixer until fluffy; add eggs, 1 at a time, beating well after each addition. Stir in flour and buttermilk. Add vanilla, stirring well. Spoon filling evenly into prepared crusts.
Bake at 350° for 25 minutes. Remove from pans, and cool completely on wire racks. Dust with powdered sugar, if desired. Dollop each serving with whipped cream.