Melt the butter over medium heat, stirring constantly until it foams and turns golden brown (about 4–5 minutes), then remove from heat and let cool for 20–30 minutes.
Preheat the oven to 350°F and line a baking sheet with parchment paper.
Whisk together the cooled browned butter, coconut sugar, and maple syrup, then add the egg and vanilla and whisk until smooth.
Stir in the flour (all-purpose or 1:1 gluten-free), collagen if using, baking soda, and salt until just combined, then fold in the chopped pistachios and dark chocolate.
Scoop dough onto the prepared baking sheet (about 2 Tablespoons each), chilling for 15–20 minutes first if the dough feels soft.
Bake for 10–13 minutes until the edges are golden and the centers look slightly underdone. Sprinkle with flaky sea salt and cool on a rack before serving.
Notes
For thicker cookies, chill the dough for 30 minutes before scooping.