Preheat your oven to 350°F (175°C) to ensure even baking.
In a large mixing bowl, melt the 10 tablespoons of unsalted butter. Let it cool slightly.
Add 2 cups of light brown sugar and 1 teaspoon of pure vanilla extract to the melted butter. Whisk until smooth.
Whisk in 3 beaten eggs until the mixture is thick and glossy.
In a separate bowl, sift together 3 cups all-purpose flour, 1 teaspoon baking powder, and a large pinch of salt.
Gradually fold the dry ingredients into the wet ingredients using a spatula until just combined.
Fold in 1 cup chopped walnuts and 1 cup semisweet chocolate chunks evenly throughout the batter.
Pour the batter into a greased or parchment-lined 9x13 inch baking pan and spread evenly.
Bake for 25-30 minutes, until edges are golden brown and a toothpick inserted in the center comes out clean.
Let the blondies cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before cutting into squares.