Combine the first 5 ingredients in a small, heavy saucepan over medium heat. Heat to 180° or until thick (about 6 minutes), stirring constantly with a whisk; remove from heat. Add butter and rind, stirring until butter melts. Place curd in a medium bowl. Cover surface of curd with plastic wrap, and chill completely. Spoon 2 tablespoons curd into each of 8 small cups. Top each serving with 1 1/2 tablespoons blueberries and 1 1/2 tablespoons raspberries. Repeat layers of curd and berries. Top each serving with 1 teaspoon crumbs.