Preheat oven to 350 degrees. Coat a 9 x 13 inch pan with cooking spray and line with parchment, set aside.
In a large bowl, stir together the melted butter and sugar until well combined. Add eggs and vanilla and stir. Stir in the flour, baking powder, and salt until well combined. Fold in the walnuts. Pour into prepared pan using spatula or damp hands to spread evenly.
Bake for 30-35 minutes until sides show signs of pulling away from pan. If blondies start to get too dark while baking, lightly cover with foil. Cool on a wire rack.
While blondies cool, make the frosting. Combine the butter, brown sugar, and milk in a saucepan. Bring to a boil and simmer for two minutes. Remove from heat and cool. Add powdered sugar and stir well. If needed add more milk to make a smooth, spreadable frosting. Spread over cooled blondies.
Cool completely and cut into about squares.
The blondies can be made ahead with excellent results. Once cooled completely, cut and store in an airtight container in the refrigerator for up to a week. Alternatively, you can freeze these for up to two months.
Recipe courtesy of This Delicious House. Sign up for more recipes today at https://thisdelicioushouse.com