Get the oven going. Preheat to 350°F. Line a 9x13-inch baking pan with parchment paper, making sure it hangs over the sides so you can lift the whole thing out later. This is the difference between perfect bars and a frustrating disaster.
Brown that butter. Melt the butter in a light-colored saucepan over medium heat. Keep stirring and watch it closely. It'll foam up, then the foam will subside and the butter will start turning golden brown with little brown bits at the bottom. When it smells nutty and amazing, take it off the heat. This should take about 5 minutes. Pour it into a large mixing bowl and let it cool for a few minutes.
Mix the wet stuff. Whisk the brown sugar into the browned butter until everything's combined and looks like wet sand. Add the eggs, egg yolks, and vanilla. Whisk until smooth and glossy. Your arm might get tired. That's normal.
Add the dry stuff. Dump in the flour, baking powder, and salt. Use a silicone spatula to fold everything together until no streaks of flour remain. Don't overmix or you'll end up with tough blondies. Just fold until it's combined.
Candy time. Gently fold in the chopped candy bars. Mix just until they're evenly distributed. Some pieces will sink to the bottom and that's fine. Life isn't perfect and neither are blondies.
Bake until golden. Scrape the batter into your prepared pan and spread it to the edges. It'll be thick and that's exactly right. Bake for 25 to 30 minutes, until the edges are set and golden brown. The middle should still look slightly underdone. Trust me on this.
Cool completely before cutting. Put the pan on a cooling rack and walk away for at least an hour. I know it's hard. Do it anyway. When they're completely cool, lift them out using the parchment paper overhang and cut into bars.