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.
Fill each cupcake using 1 1/2 cups blackberry jam. To fill cupcakes, insert the end of a wooden spoon or dowel into the center of the cupcake to make a hole. Fill a zip-top plastic freezer bag with filling or frosting. Use scissors or kitchen shears to snip about 1/4 inch from one corner of the bag; insert bag into the hole in the cupcake. Squeeze gently until filling comes to the top of each cupcake.
To prepare Vanilla Frosting, beat first 4 ingredients at medium speed with an electric mixer until creamy. Gradually add powdered sugar, beating at low speed until blended. Beat at high speed 2 minutes or until creamy. Make 3 cups.
Stir remaining 1/4 cup jam into Vanilla Frosting. Frost each cupcake with flavored frosting.
Toss together blackberries and sugar. Top each cupcake with blackberries just before serving.