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
Preheat oven to 350°F. Line 10 standard (1/3-cup) muffin cups with paper liners.
Combine butter, 1/2 cup chocolate chips, and unsweetened chocolate in top of double boiler set over simmering water. Stir until mixture is melted and smooth.
Remove from over water. Whisk both sugars into chocolate mixture, then whisk in eggs 1 at a time.
Whisk in vanilla, then flour, walnuts, salt, and remaining 3/4 cup chocolate chips.
Divide batter among prepared muffin cups (about 1/4 cup for each).
Bake cupcakes until tester inserted into center comes out with moist crumbs attached, about 20 minutes.
Transfer cupcakes to rack and cool completely.