Prepare the Cupcake Batter:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
2. Mix Dry Ingredients: In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
3. Cream the Butter and Sugar: In a large bowl, beat the softened butter and sugar together until light and fluffy (about 3-4 minutes).
4. Add the Eggs and Vanilla: Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
5. Add Dry Ingredients and Milk: Gradually add the dry ingredients to the butter mixture, alternating with the milk. Start and end with the dry ingredients. Mix until just combined.

Make the Raspberry Swirl:

6. Prepare the Raspberry Puree: If using fresh raspberries, mash them with a fork or blend them into a puree. You can add a little lemon juice to enhance the raspberry flavor if desired.
7. Swirl the Raspberry Puree: Add a small spoonful of the raspberry puree to the cupcake batter. Use a toothpick or knife to gently swirl the puree into the batter, creating a marbled effect.

Fill the Cupcake Liners:

8. Spoon the Batter into the Cups: Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
9. Add More Raspberry Puree (Optional): If you have extra raspberry puree, add a small dot on top of each cupcake and swirl again for a more pronounced raspberry swirl.

Bake the Cupcakes:

10. Bake: Place the muffin tin in the oven and bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
11. Cool: Let the cupcakes cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.