To prepare Butter Cake, preheat oven to 350 degrees. Beat butter and sugar at medium speed with an electric mixer until creamy. Add eggs, 1 at a time, beating until blended after each addition. Combine flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in vanilla.
Place paper baking cups in 2 (12-cup) muffin pans, and coat with cooking spray; spoon batter into cups, filling two-thirds full. Bake for 12-15 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans to wire racks, and cool completely.
To prepare Pecan Pie Frosting, whisk together first 6 ingredients in a heavy saucepan. Bring to a boil over medium heat, whisking constantly. Boil, whisking constantly, 1 minute or until thickened. Remove pan from heat; stir in butter and vanilla, whisking until butter melts. Place a sheet of plastic wrap directly on surface of mixture (to prevent a film from forming); chill about 3 hours or until thick. Makes 4 cups. Frost each cupcake.
To prepare Piecrust Leaves, unroll piecrust on a flat surface. Using a 1/2-inch cookie cutter, cut out leaves. Bake according to package directions. Makes about 48 leaves.
Top each cupcake with toasted pecans and 1 piecrust leaf.