In a large bowl, stir together the flour, brown sugar, cocoa powder, baking powder, baking soda, and salt. In a medium bowl, stir together the oil, buttermilk, eggs, sour cream, and vanilla extract.

Gently stir the wet ingredients into the dry ingredients. Once the mixture is nearly mixed but a few pockets of flour remain, add in the chopped chocolate and stir until everything is just combined. Cover the bowl with plastic wrap and let it rest on the counter at room temperature for about 30 minutes.

Meanwhile, preheat your oven to 375 degrees, and line a 12 cup muffin tin with paper liners. Once the batter has rested, scoop it evenly into each muffin tin. They should be fairly full.

Take about 1-2 tsp. of tahini and spoon it into each muffin cup on top of the batter. Use a toothpick to gently swirl it into the batter. Bake the muffins for 20-22 minutes, or until a toothpick inserted into the center comes out mostly clean (aside from any melted chocolate chunks). Repeat the process with the remaining batter. Cool the muffins and enjoy!