Brownie Bottom Pudding Pie

4 squares Baker's semi-sweet baking chocolate
    or 1 (4 ounce) package Bakers' German's sweet baking chocolate
1/4 cup (1/2 stick) butter or margarine
3/4 cup granulated sugar
2 eggs
1 teaspoon vanilla extract
1/2 cup flour
1/2 cup chopped nuts (optional)
2 1/2 cups cold milk
2 packages (4 serving size) Jell-0 chocolate flavor instant
    pudding & pie filling
Thawed Cool Whip