Preheat the oven to 350 degrees F (175 degrees C). Grease a 10x15-inch baking pan.

Whisk flour, baking powder, and salt together in a bowl.

Melt shortening in a saucepan over low heat or in the microwave. Stir in brown sugar until well combined; let cool slightly.

Beat eggs into the cooled brown sugar mixture until smooth. Stir in vanilla.

Add the flour mixture and stir just until combined. Fold in chocolate chips and walnuts.

Spread batter evenly into the prepared pan.

Bake until the top is golden brown and a toothpick inserted near the center comes out with a few moist crumbs, about 30 minutes.

Let cool completely in the pan before cutting into 35 squares.