Preheat oven to 350°F (175°C). Line a 9x13-inch baking pan with foil or parchment, letting excess hang over the sides for easy lifting. Lightly grease the lining.

In a small saucepan, melt the butter over low heat (or microwave in 20-second bursts). Remove from heat and whisk in the sugar until glossy.

Add the eggs one at a time, whisking after each until smooth. Stir in the vanilla.

Sift the cocoa powder, flour, salt, and baking powder over the wet mixture. Fold gently with a spatula until just combined — don’t overmix. The batter should be thick and fudgy.

Spread about two-thirds of the brownie batter into the prepared pan, smoothing the top with an offset spatula. Save the remaining batter for the top layer.

In a medium bowl, beat the peanut butter, powdered sugar, softened butter, and vanilla with a fork or paddle until creamy and spreadable.

If it’s too thick, add 1 teaspoon of milk at a time until it reaches a spreadable texture.

Dollop spoonfuls of the peanut butter mixture across the brownie layer. Use the back of a spoon to gently spread, leaving gaps so chocolate shows through.

Drop spoonfuls of the reserved brownie batter over the peanut butter layer. Use a knife or skewer to gently swirl the two layers together for a marbled effect.

Bake for 25–30 minutes, or until the top is set and a toothpick inserted near the center comes out with a few moist crumbs. Oven times vary; start checking at 22 minutes.

Let the bars cool in the pan for 20 minutes. For a shiny finish, warm the chocolate chips and cream together in 20-second bursts, stirring until smooth.

Spread over the cooled bars and sprinkle lightly with sea salt.

Refrigerate for at least 1 hour to set the swirl and glaze. Use the parchment overhang to lift the bars from the pan, then slice into squares with a sharp knife warmed briefly under hot water for cleaner cuts.