Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.

Mix brown sugar, flour, walnuts, butter, eggs, baking powder, vanilla extract, and salt together in a bowl by hand until smooth. Spread evenly into the prepared pan.

Bake in the preheated oven until the top is dry and edges have started to pull away from the sides of the pan, 25 to 30 minutes.

Remove from the oven and let cool for 10 to 15 minutes before slicing.