Preheat oven to 350°F/180°C. Grease a 9x13 baking pan and line with parchment paper.
In a medium bowl, whisk together the flour and baking powder. In a small saucepan, melt the butter. Let the butter begin to brown, once it gives off a nutty, toasty smell with brown flecks at the bottom, take it off the heat. Add the sugars, vanilla and salt and combine. Let it cool to room temperature.Add the eggs and whisk until combined.
Pour the butter mixture into a large bowl and add the flour mixture. Stir until just combined – don't overmix it!
Fold in the sprinkles, but keep ¼ cup to the side to sprinkle on top. Pour half of the batter into the pan, then lay out the oreos. Pour the remaining batter on top and sprinkle the rest of the sprinkles.
Bake for 22-25 minutes until the edges are set and the top is golden. When you insert a toothpick, you should still have a few crumbs. We don't want to overbake it. Store blondies in an airtight container for up to 4 days.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method.Room temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about 2 hours before you start baking.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Sprinkles: Make sure to use long strand sprinkles. If you use nonpareils (the little circular ball sprinkles) they'll bleed into the batter and you won't see them properly when they're baked. Storage: keep in an airtight container for up to 4 days. You can freeze the bars for up to 3 months. Let them defrost in the fridge overnight before eating.