In a saucepan over medium heat, melt the unsalted butter. Stir frequently until it turns a lovely golden brown and develops a nutty aroma. Be careful not to burn it! Once browned, remove from heat and let it cool slightly.
In a mixing bowl, add the warm brown butter, brown sugar, and honey. Whisk together until well combined. The warmth of the butter will help dissolve the sugar.
Add in the large egg and the egg yolk to the mixture, whisking until fully incorporated. This will add richness and moisture to your cookie bars.
Sprinkle the flaky sea salt and all-purpose flour over the wet mixture. Use a spatula or wooden spoon to gently fold the flour into the mixture until just combined. Avoid overmixing.
Gently fold in the chopped pistachios, ensuring they are evenly distributed throughout the batter.
Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang for easy removal later.
Transfer the batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for about 20-25 minutes or until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Once baked, remove the pan from the oven and let the cookie bars cool in the pan for about 10 minutes. Lift them out using the parchment paper and let them cool completely on a wire rack before slicing into bars.