Preheat your oven to 350°F (175°C). Prepare your baking pan by lining it with parchment paper, allowing some overhang for easy removal later.
In a large mixing bowl, combine the unsalted butter, light brown sugar, and creamy peanut butter. Use a whisk or electric mixer to cream these ingredients together until light and fluffy.
Beat in the large eggs, one at a time, ensuring each is fully incorporated before adding the next. Stir in vanilla extract until well combined.
In another medium bowl, whisk together all-purpose flour, old-fashioned oats, baking powder, and salt until evenly mixed.
Gradually fold the dry ingredients into the wet mixture using a spatula. Mix until just combined; be careful not to overmix.
Fold in semi-sweet chocolate chips and the halved Snickers bars, ensuring they are evenly distributed throughout the batter.
Spread the batter evenly into the prepared baking pan. Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean.
Allow the cookie bars to cool in the pan for about 10 minutes, then lift them out using the parchment overhang. Let them cool completely on a wire rack before slicing into squares.