Preheat your oven to 400°F (200°C) and prepare a muffin pan with non-stick spray or liners.

In a large bowl, whisk together the flour, baking powder, and salt until combined.

In a medium bowl, whisk the brown sugar, granulated sugar, and egg until smooth, then add buttermilk, oil, and vanilla, mixing well.

Pour the wet ingredients into the dry mixture and fold until just combined, avoiding overmixing.

Gently fold in raspberries and dark chocolate.

Divide the batter evenly into the muffin cups, filling them to the top.

Bake in the preheated oven for about 25 minutes until the tops are golden brown and a toothpick inserted comes out clean.

Let muffins cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely.