Preheat oven to 375 degrees F. Place almonds in a blender or food processor. Cover and blend or process until ground. In a small bowl stir together ground almonds and the 3 tablespoons sugar. Drizzle with melted butter; toss gently to coat. Press mixture evenly onto bottom and up side of a 9- to 9-1/2-inch tart pan that has a removable bottom. Bake about 12 minutes or until golden. Cool on a wire rack.
In a small saucepan cook and stir chopped chocolate over low heat until melted. Spread melted chocolate over bottom of crust; set aside.
For filling, in a medium saucepan heat the 1/2 cup butter over medium heat until melted. In a small bowl combine the 3/4 cup sugar and cornstarch; stir into melted butter. Stir in grapefruit peel, grapefruit juice, orange juice, and cream. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Gradually stir about 1 cup of the hot mixture into egg yolks. Add egg yolk mixture to saucepan. Bring to a gentle boil; reduce heat. Cook and stir for 2 minutes.
Place saucepan in a large bowl of ice water and stir frequently until filling is cool. Pour filling into baked crust, spreading evenly. Cover surface with plastic wrap and chill for 3 to 4 hours or until set.
To serve, remove side of pan. Cut tart into wedges. Top each serving with whipped cream and grapefruit sections and/or cocoa powder. Makes 12 servings.