Preheat your oven to 175°C (350°F). Grease your 9x9 inch pan or line it with parchment paper.
In a large mixing bowl, combine the melted butter, peanut butter, brown sugar, and vanilla extract. Whisk everything together until the mixture is smooth, fragrant, and slightly glossy.
In a separate bowl, stir together the oats, flour, baking powder, and salt until evenly blended.
Gradually add the dry mixture to the wet ingredients, stirring gently until just combined. The dough will be thick and slightly sticky.
Fold in the chocolate chips, distributing them evenly throughout the dough. You should see pockets of melty chocolate spots.
Transfer the dough into your prepared pan and press it down firmly with the back of a spatula or your wet fingers to create an even layer. This helps ensure a chewy, dense texture.
Bake the bars in the preheated oven for 15–18 minutes, or until the edges turn a golden brown and the center looks slightly soft but set.
Remove the pan from the oven and let the bars cool in the pan for about 10 minutes. This helps them firm up and makes slicing easier.
Transfer the bars onto a cooling rack and let them cool completely. Once cooled, cut into squares or rectangles, roughly 2 inches wide.
Enjoy these chewy bars as a quick snack or sweet treat, storing any leftovers in an airtight container at room temperature for up to three days.