1. Preheat the oven to 350°F and line an 8×8-inch baking pan with parchment paper.

2. Melt the butter in a light-colored saucepan over medium heat, stirring often, until it smells nutty and brown bits form at the bottom. Remove from the heat and cool for 10 to 15 minutes.

3. Pour the browned butter into a mixing bowl, including the toasted bits. Add the brown sugar, honey, and granulated sugar, then whisk until glossy.

4. Whisk in the eggs one at a time, then stir in the vanilla extract.

5. In a separate bowl, combine the flour, baking powder, and salt.

6. Add the dry ingredients to the wet ingredients and stir just until combined.

7. Fold in the chopped pistachios and white chocolate chips if using.

8. Spread the batter evenly into the prepared pan and smooth the top. Add a few extra pistachios on top if desired.

9. Bake for 24 to 30 minutes, until the top is lightly golden and a toothpick inserted near the center comes out with a few moist crumbs.

10. Cool completely in the pan, sprinkle with flaky sea salt if desired, then slice into bars.