In a mixing bowl, cream butter and sugar. Add eggs, one at a time, beating well after each addition. Add milk and vanilla; beat until light and fluffy. Combine flour and baking powder; gradually add to creamed mixture and mix well. Chill at least 2 hours or overnight. On a lightly floured surface, roll the dough to 1 cm thickness. Cut with trefoil cookie cutter or cutter of your choice. Place cookies on ungreased baking pans. Sprinkle with sugar if desired. Bake at 180ºC or 8-10 minutes until lightly browned. Cool on wire racks.