Coarsely shred 2 1/2 ounces almond paste to make about 1/2 cup. In a bowl, stir almond paste and 2 tablespoons melted butter to form a soft paste. Slice open 4 fresh croissants. Spread almond paste mixture equally on cut sides of bottom halves, then sprinkle 1/4 cup (1 oz.) coarsely chopped semisweet or bittersweet chocolate equally over almond paste. Replace tops. Set croissants on a baking sheet and bake in a 350° oven until chocolate starts to melt, 5 to 7 minutes.