Peanut Butter & Fudge Brownies with Salted Peanuts

Brownies:
3/4 cup (1 1/2 sticks) unsalted butter
7 ounces bittersweet or semisweet chocolate, chopped
3 ounces unsweetened chocolate, chopped
1 1/2 cups sugar
1 1/2 teaspoons vanilla extract
1/4 teaspoon salt
4 large eggs
1 cup all purpose flour
1 cup roasted salted peanuts, coarsely chopped

Frosting:
1 cup chunky peanut butter (do not use natural or old-fashioned)
1/4 cup unsalted butter, room temperature
3/4 cup powdered sugar
1/8 teaspoon salt
1/8 teaspoon ground nutmeg
1 tablespoon whole milk
1 teaspoon vanilla extract

Ganache:
7 ounces bittersweet or semisweet chocolate, chopped
1/4 cup unsalted butter, room temperature

For brownies:
Position rack in center of oven and preheat to 325°F. Line 13x9x2-inch metal baking pan with foil, leaving long overhang; butter foil.

Place 3/4 cup butter in heavy large saucepan. Add both chocolates; stir over low heat until smooth. Remove from heat. Whisk in sugar, vanilla, and salt, then eggs, 1 at a time. Fold in flour, then nuts. Spread in prepared pan. Bake until tester inserted into center comes out with moist crumbs attached, about 30 minutes. Place pan on rack; cool.

For frosting:
Using electric mixer, beat peanut butter and 1/4 cup butter in medium bowl to blend. Beat in powdered sugar, salt, and nutmeg, then milk and vanilla. Spread frosting over brownies.

For ganache:
Stir chocolate and 1/4 cup butter in heavy small saucepan over low heat until smooth. Drop ganache all over frosting; spread to cover. Chill until set, about 1 1/2 hours. Can be made 1 day ahead. Cover and keep chilled.

Using foil as aid, transfer brownie cake to work surface; cut into squares. Bring to room temperature; serve.