1 1/4 cups (2 1/2 sticks) unsalted butter, plus more for pan
2 1/4 cups all-purpose flour, plus more for pan
1 1/2 teaspoons baking powder
1 1/2 teaspoons salt
2 cups packed light-brown sugar
1/2 cup granulated sugar
3 large eggs
2 1/2 teaspoons pure vanilla extract
1 cup chopped walnuts (about 4 ounces)
1 cup Heath toffee bits
Preheat oven to 350 degrees. Butter a 9X13 baking pan. Line bottom of pan with parchment paper; butter and flour parchment paper.
In a saucepan over medium heat, cook the butter until it turns golden brown; remove from heat, and let cool. Whisk together flour, baking powder, and salt. WARNING: this took me a while.
In the bowl, combine browned butter and both sugars; stir with a wooden spoon until combined. Add eggs. Using the paddle attachment, beat on medium-high speed until light and fluffy, about 3 minutes. Add vanilla, and beat to combine. Add flour mixture, walnuts, and toffee bits. Mix until thoroughly combined, and pour into prepared pan.
Bake until tester comes out clean, 35 to 40 minutes Transfer to a wire rack to cool. Peel off parchment paper and cut blondies into squares.
Read more at Marthastewart.com: Brown Butter Toffee Blondies - Martha Stewart Recipes