Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking pan.

Make the Brownie Base:

• In a medium saucepan, melt the butter and unsweetened chocolate over low heat, stirring occasionally until smooth.

• Remove from heat and whisk in the sugar and vanilla extract. Add the eggs one at a time, mixing well after each addition.

• In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until combined.

• Stir in the chopped walnuts.

• Pour the brownie batter into the prepared baking pan, spreading it evenly.

Bake the Brownies:

• Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

• Let the brownies cool in the pan on a wire rack while you prepare the topping.

Prepare the Fudge Topping:

• In a large saucepan, combine the sugar, evaporated milk, and butter. Bring the mixture to a boil over medium heat, stirring constantly. Let it boil for 5 minutes.

• Remove the pan from the heat and stir in the semisweet chocolate chips, milk chocolate chips, marshmallow creme, and vanilla extract until smooth and fully combined.

• Stir in the chopped walnuts.

Assemble the Brownies:

• Pour the fudge topping over the cooled brownie base, spreading it evenly with a spatula.

• Let the brownies sit at room temperature until the fudge topping sets, about 2 hours.

Cut and Serve:

• Once set, cut the brownies into squares and serve.