Preheat oven to 185 degrees Celsius (365 degrees Fahrenheit). Line a 12-hole muffin tin with paper liners and set aside.
Combine flour, baking powder, baking soda and salt in a small bowl. Set aside.
Place butter and sugar in a medium mixing bowl and using an electric mixer, cream the mixture on low for 1 minute, then cream on medium-high speed for another 5 to 7 minutes until light and fluffy, scrapping down the sides occasionally.
Add in the egg and beat on medium speed for 1 minute. Scrap down the sides and beat in the vanilla on medium speed for 30 seconds.
Turn the mixer speed down to low and while the mixer is running, pour the buttermilk into the batter (avoid pouring onto the beaters as the liquid will splatter). Stop the mixer when all buttermilk has been mixed in. The mixture will be liquidy and curdly.
Sift in the flour mixture and fold in the flour mixture with a spatula until around 80% of the flour mixture has been mixed in. Add in your Add-Ons and fold in gently.
Scoop the batter into the paper liners until 3/4 full, about 1/4 cup of batter each. Depending on the size of your paper liners, you may end up with 10 to 14 muffins.
Bake the muffins for 15 to 20 minutes until a skewer inserted into the centre of the muffin comes out clean and the top of the muffin springs back when gently touched. Set the muffin tin on a wire rack and let the muffins cool.
Serve warm or at room temperature.
NOTES
- If your dark brown sugar is too coarse or a clumping together, process for several seconds in a food processor before using. - To make your own buttermilk, combine 180 ml (3/4 cup) milk with 3/4 tablespoon white vinegar or lemon juice. Let stand for 10 minutes and stir well before using. - Store the muffins in an airtight container at room temperature for 2 to 3 days, but if there are fruits inside your muffins, it is better to store them in an airtight container and in the refridgerator. Warm up your muffins in a 100 degrees Celsius (212 degrees Fahrenheit) oven for about 5 minutes before serving. - Adapted from Smitten Kitchen's Perfect Blueberry Muffins