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.
According to Jan Moon at Dreamcakes, when you toss the sugar with the berries, they begin to release their sweet juice. So don't put the berries on the cupcakes until just before serving. She loves when the juice trickles down the sides and makes a puddle on the plate and suggests that you may want to eat this one with a spoon.