Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
In another bowl, beat the egg and mix in buttermilk, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in the blueberries or chocolate chips.
Do not overmix; the batter should be lumpy.
Fill each muffin cup about ⅔ full with batter.
Bake in the preheated oven for 20-25 minutes, or until a toothpick comes out clean.
Let the muffins cool in the pan for 5 minutes before transferring to a wire rack.
Enjoy warm or store for later!