Loaf-Pan Buckeye Brownies

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.

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.

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.

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.