Position rack in center of oven and preheat to 325°F. Line mini muffin tin with papers or grease tin.
Place 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.
Distribute into mini muffin tin, about 1 tablespoon per bite. Bake until tester inserted into center comes out with moist crumbs attached, about 20-25 minutes. Place pan on rack; cool.
For frosting and ganache: Using electric mixer, beat peanut butter and butter in medium bowl to blend. Beat in powdered sugar, salt, and nutmeg, then milk and vanilla.
Spread frosting over brownies.
Stir chocolate and remaining butter in heavy small saucepan over low heat until smooth. Drop ganache all over frosting; spread to cover.
Chill until set, about 1 ½ hours. Can be made 1 day ahead. Cover and keep chilled. Bring to room temperature before serving.
