Preheat oven to 400ºF. Spray 18 wells of two 12-count muffin pans with non-stick spray or line the wells with cupcake liners. Set aside.
In a large bowl, toss together the flour, baking powder, and salt. Set aside.
In a medium size bowl, whisk together the butter, granulated sugar, and brown sugar until smooth. Add the eggs, buttermilk, and vanilla extract and whisk again until everything is combined.
Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. Carefully fold the berries (reserving some for tops of the muffins-- about 1/4 cup) into the muffin batter, being aware the color may spread a bit.
Spoon batter equally into prepared muffin pans. Place additional berry pieces in the tops of the muffins if desired. Bake muffins for 5 minutes, then reduce heat to 350ºF and bake for an additional 16-18 minutes until tops start to brown and a toothpick inserted in the center comes out mostly clean. Remove from oven and allow to cool completely on a wire rack. Muffins will stay fresh in an airtight container at room temperature up to 3 days or in the refrigerator up to one week. Muffins freeze well, up to 2 months. Thaw in microwave for 45-60 seconds before eating.
Notes
It is always a good rule of thumb to use room temperature eggs when using room temperature butter. To bring eggs to room temperature quickly, place in a bowl of warm water for 10 minutes.
If you don't have buttermilk, you can easily make your own: see my post about homemade buttermilk.
You may also use fresh blueberries or a combination of fresh and frozen. I used whole blueberries, whole and halved raspberries, and chopped strawberries. Any combination of berries is fine, totaling 2 cups.
All nutritional values are approximate and provided to the reader as a courtesy. Changing ingredients and/or quantities will alter the estimated nutritional calculations.