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1
Heat the oven to 300F Grease an 8-inch square baking dish with cooking spray, line with parchment paper, and lightly spray paper to coat.
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2
Bring 2 inches water to a simmer in a medium saucepan over medium-high heat.
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3
Place the butter and chocolate in a medium heatproof bowl, then place the bowl over the simmering water ( the bottom of the bowl should not touch the water).
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4
Reduce heat to low and melt the choclate and butter together, stirring often.
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5
Remove from heat and whisk in eggs, sugar, vanilla, and salt until completely incorporated.
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6
Whisk in flour and scrape mixture into the prepared pan, using a rubber spatula to evenly spread the batter and smooth the top.
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7
Place the peanut butter in a quart size resealable plastic bag and snip off one corner, making a 1/2 inch hole.
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8
Squeeze the peanut butter down into the bag and twist top.
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9
Pipe the peanut butter in four straight lines across the top of the brownie batter.
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10
Use a toothpick to swirl the peanut butter into the batter.
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11
Bake until the the chocolate portions of the brownie have a smooth sheen and are dry, about 45 minutes (the peanut butter portion will still be sticky).
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12
Let cool completely before slicing into 2 inch squares.
