For the brownie:
5 ounces unsweetened chocolate, melted and cooled
3/4 cup (1-1/2 sticks) unsalted butter, softened
1-3/4 cups sugar
3 eggs
1 teaspoon vanilla extract
1 cup all-purpose flour
1 cup chopped walnuts (optional)
For the topping:
1-1/2 cups semisweet chocolate chips
1 cup Nutella
1 tablespoon unsalted butter
1-1/2 cups crispy rice cereal
Preheat the oven to 350 degrees F. Grease a 9" x13" x2" pan or line with foil and spray lightly with nonstick cooking spray.
In a large mixing bowl, combine the melted chocolate and butter and stir until smooth. Add the sugar and stir with a wooden spoon until well blended. Add the eggs and vanilla and mix well. Mix in the flour and nuts, if using, and stir. Pour the batter into the prepared pan.
Bake for 30 to 40 minutes, or until the edges appear to be set (the center should still be soft). Do not overbake.
While the brownies are baking, place the chocolate chips, peanut butter, and butter in a medium saucepan. Cook over low heat, stirring constantly until melted. Remove from heat, add the cereal and mix well. Allow this to cool for three minutes or so. Spread the mixture evenly over the brownies once they’re baked.
Refrigerate until chilled before cutting.