Step 1

Prepare an 8" X 8" baking pan with nonstick spray or grease with softened butter.

Step 2

In a large bowl, whisk the flour, baking powder, salt, and baking soda together; set aside.

Step 3

In a large bowl, combine the butter and brown sugar. Using a handheld mixer or a stand mixer fitted with a paddle attachment, beat together on high speed until its light and fluffy, about 3 minutes. Add the egg and vanilla and beat the batter until combined. Using a spatula, scrape down the sides of the bowl, and with the mixer on low, slowly add the flour to blend. Add the chocolate chips and chopped nuts, beating just until blended. Evenly press the dough into the pan and cover with plastic wrap. Refrigerate for 30 minutes.

Step 4

Preheat the oven to 375°F.

Step 5

Remove from the fridge and bake for 15-18 minutes, or until the dough pulls away from the sides of the pan and edges are just beginning to brown. Place on a cooling rack. When cool, slide a knife around all of the pan's edges and flip the cookie bars out onto a cutting board to cut into bars.

Step 6

Store in plastic wrap or an airtight container for 5 days.