Preheat oven to 350 degrees. Line a 9 x 13 baking dish with parchment paper, sparying the parchment with non-stick cooking spray. Set aside.
To a large saucepan over medium heat, add the butter and peanut butter. Cook, stirring until melted and smooth. Remove the pan from the heat and whisk in the sugar followed by the eggs. Stir in the 1/2 teaspoon salt, flour, and baking powder until well combined. Fold in the peanut butter and white chocolate morsels. Spread the mixture into the prepared pan and press the batter down evenly with your fingers. Sprinkle the top of the blondies with flake sea salt if using.
Bake for 30 minutes or until set. Allow to cool at least 30 minutes before slicing.