To make the blondies:
Preheat oven to 325°F. Line a 9x13 inch baking pan with parchment paper or grease with nonstick baking spray.
Melt the butter and sugar together in a medium saucepan over medium heat. Once melted, remove from heat and allow to cool for 5 minutes. Stir in the peanut butter until combined. Add the eggs one at a time, and then the vanilla, stirring until combined.
In a separate bowl, combine the flour, baking powder, and salt. Add to the wet ingredients and stir until combined. Spread the batter into the prepared baking sheet. Bake for 30-40 minutes, until a toothpick inserted into the center comes out mostly clean.
Let cool completely before frosting.
To make the buttercream:
Combine all ingredients in the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until smooth, scraping down the mixer bowl a few times as needed.
Frost blondies with buttercream. Slice into squares and serve.
NOTES:
- To melt chocolate, microwave chocolate in a heat-proof bowl on high in 30 second intervals. Stirring until smooth.
Calories: 318kcal, Carbohydrates: 41g, Protein: 5g, Fat: 16g, Saturated Fat: 7g, Cholesterol: 38mg, Sodium: 137mg, Potassium: 163mg, Fiber: 1g, Sugar: 31g, Vitamin A: 265IU, Calcium: 39mg, Iron: 1mg