Prepare brownies according to box directions and allow to cool.
Remove from pan (in 1 big sheet) and cut as many heart shapes as possible with cookie cutter.
Puree raspberries in food processor or blender. (if using frozen raspberries, make sure that they are completely thawed and drained of excess juice)
In a medium bowl, cream together the butter and raspberry puree.
Add in powdered sugar 1 cup at a time, mixing until smooth and creamy.
Transfer filling to a piping bag.
Pipe filling around the edge of one of the brownie hearts, then fill in the center with more filling. Top with another brownie heart, gently pressing down to make a sandwich.
Dust the tops with additional powdered sugar.