In the bowl of a stand mixer, cream together the butter and sugar. Whip them on high speed until the mixture's light & fluffy.
Add the egg, and continue whipping on high speed again until the batter's light and fluffy.
In a separate mixing bowl, add the dry ingredients- the flour, baking powder, soda, and salt. Using a whisk, gently stir them until evenly combined.
Using a spatula, scrape the batter down the sides of the mixing bowl adding it back to the main mixture.
Reduce the mixing speed to the lowest setting and begin adding the rest of the ingredients. Start with the flour mixture, then the buttermilk and mix until completely combined. Repeat, alternating and incorporating them but ending with the buttermilk.
Liberally spray a cupcake pan or muffin tin with non stick cooking spray. To each bowl, add roughly 3 tablespoons of batter.
Bake the muffins at 350 degrees for roughly 20 minutes, or until a tooth pick inserted in the center comes out clean.
Remove the muffins from the oven, let them rest for 5 full minutes, and then run a butter knife around the edges to release them from the pan. Transfer them to a wire rack and let them finish cooling.