Heat the oven to 350°F and line a small 8x4 inch loaf pan with parchment paper.
In a mixing bowl, stir together the peanut butter, melted butter, and brown sugar until smooth.
Add the egg and vanilla extract, then mix until fully combined.
Sprinkle in the flour, baking powder, and salt. Stir until no dry streaks remain.
Spread the batter evenly in the prepared pan and bake for 18-20 minutes until the edges look set.
While the blondies cool, melt the chocolate chips in a small bowl. Spoon the melted chocolate into a piping bag or zip-top bag with a corner snipped, then pipe a spiderweb pattern across the top. Let the chocolate set before slicing.