Chill the buttermilk. Measure buttermilk into a liquid measuring cup and place in the freezer for about 15 minutes while you prep.
Brown the butter. Melt butter in a light-colored saucepan over medium heat. Cook, swirling occasionally, until foamy, golden brown, and nutty-smelling, about 5-7 minutes. Immediately pour into a heatproof bowl, scraping out all the brown bits. Set aside about 3 tablespoons for brushing. Let cool 5-10 minutes until warm but not hot.
16 tablespoons unsalted butter
Mix dry ingredients. Whisk together flour, baking powder, baking soda, salt, and brown sugar in a large bowl. Stir in the sage and thyme until evenly distributed.
2 cups all-purpose flour, 1 tablespoon baking powder, ½ teaspoon baking soda, 2 teaspoons kosher salt, 2 tablespoons light brown sugar, 2 teaspoons fresh sage, ¾ teaspoon fresh thyme leaves
Preheat oven to 425°F. Line a baking sheet with parchment paper.
Combine wet ingredients. Pour cold buttermilk into the warm brown butter and stir to combine.
¾ cup buttermilk, 10 tablespoons brown butter from above
Make the dough. Pour wet mixture into dry ingredients and stir with a rubber spatula until just combined. Dough will be shaggy and sticky — don't overmix!
Scoop and chill. Drop dough by 2-tablespoon scoops onto prepared baking sheet, spacing 1½ inches apart. You should get 14-16 biscuits. Pop the baking sheet in the freezer while the oven finishes preheating, about 10-15 minutes.
Bake for 10-12 minutes, until tops are golden and edges are lightly browned.
Brush and serve. Let cool 2-3 minutes, then brush tops with reserved brown butter (or melted butter). Top with flaky salt if desired. Serve warm!
3 tablespoons reserved brown butter (or melted butter), flaky salt