Making Key Lime Pie Bars – Browned Butter Blondie can be straightforward if you carefully follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, allowing some overhang to help with lifting out the bars later.

2. Brown the Butter: In a medium saucepan over medium heat, melt the butter. Continue to cook, swirling occasionally until it turns golden brown and smells nutty. Remove from heat and let it cool for a few minutes.

3. Mix the Blondie Base: In a large mixing bowl, combine the browned butter, brown sugar, and granulated sugar. Mix well until smooth. Then, add in eggs one at a time, mixing thoroughly after each addition, followed by the vanilla extract.

4. Add Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually fold the dry mixture into the wet mixture until fully incorporated.

5. Bake the Blondie Layer: Spread the blondie batter evenly into your prepared baking pan. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted in the center comes out mostly clean.

6. Prepare the Key Lime Filling: While the blondie layer is baking, whisk together the sweetened condensed milk, key lime juice, egg yolks, lime zest, and vanilla extract in a mixing bowl until completely combined.

7. Bake Again: Once the blondie base is ready, remove it from the oven and pour the key lime filling over the top layer. Return it to the oven for an additional 15-20 minutes, or until the filling is set. The edges should be slightly puffed but the center should still jiggle a bit.

8. Cool and Chill: Let the bars cool in the pan for about 10 minutes, then transfer to the refrigerator for at least 2 hours to chill completely.

Once they are chilled, these beauties are ready to slice and serve!