Preheat oven to 375 F (190 C). Grease or line a 12 cup muffin pan.Bring eggs and buttermilk to room temperature. In large bowl, whisk together flour, brown sugar, baking powder, baking soda, and salt.
In separate bowl, whisk together eggs, buttermilk, and vanilla. Whisking well, add oil in a slow stream until fully incorporated.
Pour over dry ingredients.
Sprinkle over chocolate chips.
Using rubber spatula, fold batter to mix just until flour is fully incorporated. Don't over mix batter!
Spoon batter into greased muffin cups.
Bake for 20 to 25 minutes or until tops are firm to the touch. Let pan cool on rack for a few minutes before removing muffins to rack to cool.