Preheat oven to 425°F (220°C). Line a standard 12-cup muffin pan with paper liners or spray generously with nonstick cooking spray.

In a large mixing bowl, whisk together flour, granulated sugar, baking powder, and baking soda until well combined.

In a medium bowl, whisk together buttermilk, melted butter, eggs, and vanilla extract until smooth.

Pour the wet ingredients into the bowl with the dry ingredients. Stir the mixture just until combined. The batter will be thick and slightly lumpy; do not overmix.

Gently fold in most of the raspberries, reserving a few for the tops.

Let the batter rest at room temperature for 15 minutes. This helps hydrate the flour and supports taller muffin tops.

Place the muffin pan in the oven and bake at 425°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F (175°C) and continue baking for 14 to 17 minutes, or until a toothpick inserted into the center of a muffin comes out with a few moist crumbs but no wet batter.

Remove the muffin pan from the oven and let the muffins cool in the pan for 5 minutes. Then transfer them to a wire rack to cool completely.

Store muffins in an airtight container at room temperature for up to 3 days. Muffins can also be frozen for up to 2 months.