Preheat your oven to 350 degrees Fahrenheit. This ensures a perfectly baked bar right from the start.
Line a 13" x 9" pan with aluminum foil or parchment paper. This step is crucial for easy removal of the bars once they’re baked.
In a large mixing bowl, stir together 1.5 cups of all-purpose flour, 1.5 cups of old-fashioned oats, 1 teaspoon of baking powder, and ¼ teaspoon of salt. Mixing these dry ingredients first helps distribute the baking powder evenly.
In another mixing bowl, beat together ½ cup of unsalted butter and ¼ cup of smooth peanut butter with an electric mixer for about 30 seconds until creamy.
Add in 1 cup of packed light brown sugar and ½ cup of granulated sugar to the butter and peanut butter mixture. Beat until the mixture is light and fluffy, which should take around 3 minutes.
Add in the 2 large eggs, one at a time, beating well after each addition. This incorporation is important for a smooth batter.
Gradually add the flour mixture in two additions, mixing well after each addition until just combined. Be careful not to overmix, as this can make the bars dense.
Fold in 1.5 cups of semi-sweet chocolate chunks or chips until evenly distributed throughout the batter. The chocolate will melt slightly as the bars bake, creating gooey pockets of sweetness.
Spread the dough into the prepared pan, smoothing it out with a spatula for even baking.
Bake for 23 to 26 minutes, or until the center is set and the edges are slightly browned. Keep an eye on them as they bake to avoid overcooking.
Remove from the oven and allow to cool in the pan for a bit before lifting them out using the foil or parchment. Cut into squares and enjoy! Keep them stored in an air-tight container for up to five days.