Step 1

Preheat oven to 350°F. Sift confectioners' sugar into a bowl. In a small, heavy saucepan, heat peanut butter, 1 tablespoon butter, milk, and pinch of salt over medium-low, stirring often, until just melted and combined. Pour over confectioners' sugar and stir to combine.

Step 2

Butter a standard (8 1/2-by-4 1/2-inch) loaf pan and line with parchment, leaving a slight overhang on long sides. Melt remaining 4 tablespoons butter and 2/3 cup chocolate chips in a heat-proof bowl set over (but not in) a pot of simmering water, stirring until smooth, 3 to 4 minutes. Remove from heat and add granulated sugar; whisk vigorously 1 minute. Whisk in egg vigorously until mixture is glossy, another 1 minute.

Step 3

Whisk in cocoa, vanilla, and remaining 1/2 teaspoon salt. Fold in flour and remaining 1/3 cup chocolate chips. Dollop half of brownie batter into prepared pan and spread with a spoon or offset spatula to reach edges. Dollop peanut-butter mixture over top and spread in an even layer. Top with remaining brownie batter and spread to reach edges.

Step 4

Bake until center is set and edges pull away from pan, about 30 minutes. Let cool completely, about 1 hour, before lifting out of pan and slicing. Brownies can be stored in an airtight container at room temperature or refrigerated up to 3 days.