In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, peanut butter, and brown sugar together until smooth, about 30 seconds. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla. Add the flour and baking powder and stir until just combined, don't overmix.

Pour half of the batter into the prepared pan, smoothing out the top. Measure out Nutella in a glass measuring cup and microwave for 15 to 20 seconds, then spread evenly over the batter. Top with remaining batter by dropping spoonfuls evenly over the Nutella and gently smoothing out the top.

Bake for 23 to 25 minutes, or until the top is no longer glossy and the edges are golden brown. Cool completely before slicing. Blondies will keep airtight at room temperature for up to 1 week, or in the freezer for up to 6 months.