Preheat the oven to 350°F. Grease an 8-inch pan or line it with parchment paper, leaving an overhang to lift the bars out later if desired.

If the 1 cup peanut butter (or allergy-friendly alternative) is stiff, warm it gently until stirrable. Place the peanut butter in a medium bowl.

Add 1/4 cup applesauce (or yogurt such as coconut-milk yogurt) and 1 tsp pure vanilla extract to the peanut butter and stir until smooth and combined.

In a separate bowl, combine 1 1/2 tsp baking soda, 1/8 tsp salt, 1/4 cup plus 2 tbsp flour (50 g), and 2/3 cup granulated sugar (or xylitol). Stir in 1/4 cup chocolate chips, if using.

Pour the dry ingredients into the wet ingredients and stir to combine. The mixture will be dry at first—work to break up pockets of peanut butter and scrape the bowl as needed so everything incorporates.

If the mixture remains crumbly, slowly add up to 2 tbsp milk of choice, a little at a time, just until the batter comes together into a stiff, cohesive dough. Do not add more than 2 tbsp.

Transfer the batter to the prepared pan and press it down evenly to form a flat layer. If you like, place a second sheet of parchment on top and press to smooth, then remove the top parchment. Press extra chocolate chips into the top if desired.

Bake for 10 minutes. The top should look set even if the interior seems undercooked—that is expected.

Remove the pan from the oven and let the bars cool in the pan on a wire rack for at least 20 minutes, until firm enough to cut. They will firm up further as they cool (and even more overnight).

Lift the bars from the pan using the parchment overhang (if used) and cut into pieces.