Preheat the oven to 425F and line a 12-cup muffin pan with 6 muffin liners.

In a saucepan over medium-high heat, melt and slightly brown the butter (about 5 minutes), then pour it into a large bowl to cool slightly.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt; set aside.

Whisk the cooled brown butter with avocado oil, sugar, and eggs until combined. Then add Greek yogurt, buttermilk, and vanilla, whisking until well combined.

Fold the dry ingredients into the wet, mixing until just combined. Fold in the mini chocolate chips.

Cover the batter and let it rest for 15 minutes.

Divide the batter between the muffin cups, filling them to the top and almost overfilling.

Sprinkle the tops with turbinado or coarse sugar.

Bake at 425F for 7 minutes, then reduce the temperature to 350F and bake for an additional 12-15 minutes, or until the tops are lightly golden and a toothpick comes out clean.

Allow the muffins to cool in the pan for 10 minutes before serving warm with butter and flaky sea salt.