Brownie Cupcakes With Peanut Butter Frosting

Ingredients

• 6 tablespoons (3/4 stick) unsalted butter, cut into 4 pieces
• 1 1/4 cups semisweet chocolate chips (about 8 ounces)
• 3 ounces unsweetened chocolate, chopped
• 1/2 cup (packed) golden brown sugar
• 1/3 cup sugar
• 2 large eggs
• 1 teaspoon vanilla extract
• 1/2 cup all purpose flour
• 1/3 cup walnuts, toasted, chopped
• 1/4 teaspoon salt
• 1 cup powdered sugar
• 3/4 cup creamy peanut butter (do not use old-fashioned or freshly ground)
• 1/4 cup (1/2 stick) unsalted butter, room temperature
• 1/4 teaspoon vanilla extract
• 4 teaspoons (about) whipping cream (optional)
• chocolate shavings or chocolate sprinkles