Fudgy Frosted Brownies with Walnuts

Preheat the oven to 350ºF (177ºC). Line an 8" or 9" square metal baking pan² with foil and spray with nonstick spray. Set aside.
In a medium saucepan over low heat, melt the butter and chopped chocolate, stirring frequently until completely smooth. Allow to cool for about 10 minutes.
½ cup (113g) unsalted butter, 8 ounces (227g) semi-sweet chocolate
Whisk both sugars into slightly cooled chocolate mixture until combined. Add the eggs one at a time, whisking after each addition until smooth. Whisk in the vanilla extract. Add the flour and salt and stir gently until the batter mostly comes together, then add the walnuts and stir until completely combined.
¾ cup (150g) firmly packed light brown sugar, ¼ cup (50g) granulated sugar, 3 large eggs, 1 teaspoon vanilla extract, ¾ cup (90g) all-purpose flour, ¼ teaspoon salt, 1 cup (113g) finely chopped walnuts
Pour the batter into the prepared pan. Bake brownies for 25-30 minutes or until a toothpick or cake tester inserted in the center comes out with moist crumbs. Allow brownies to cool completely before frosting.

In a small saucepan⁴ over medium heat, melt the unsweetened baking chocolate and the butter, stirring constantly to prevent scorching or burning. Remove from heat and allow to cool for 15 minutes.
2 ounces (57g) unsweetened baking chocolate3, ¼ cup (57g) unsalted butter
In a medium size bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine the cooled chocolate/butter mixture, vanilla extract, and milk or cream, starting with 2 Tablespoons (30mL).
1 teaspoon vanilla extract, 2 to 4 Tablespoons (30-60mL) milk or cream
Turn the mixer down to low and slowly add the powdered sugar and salt. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. If it starts to clump, just keep mixing. It will work itself out.
2 cups (240g) powdered sugar, ⅛ teaspoon salt
Add more liquid as needed to reach desired consistency. Buttercream is ready to use right away.
When you are ready to frost the brownies, remove the brownies by lifting the foil overhang straight up. Peel back remaining foil and place brownie block onto a large cutting board or plate.
Spread the frosting evenly onto the top of the brownies using an offset spatula, then sprinkle with additional walnuts, if desired. You can slice the brownies right away or allow the frosting to crust a bit. Store leftover brownies at room temperature up to 5 days. Brownies freeze well (frosted or unfrosted), up to 3 months. Thaw in refrigerator overnight.