Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a mixing bowl, whisk together the melted butter, brown sugar, and maple syrup until smooth.
Add the eggs and vanilla extract, and whisk until well combined.
In another bowl, mix the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the chopped walnuts.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow to cool in the pan before cutting into squares.