Prepare the Raspberry Swirl:

1. In a small saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat, stirring often, until the raspberries break down and the mixture thickens (about 5–7 minutes).
2. Strain the mixture through a fine-mesh sieve to remove seeds and let it cool completely.

Make the Cupcakes:

1. Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with cupcake liners.
2. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
3. In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Mix in vanilla, lemon zest, and lemon juice.
4. Gradually add the dry ingredients to the wet mixture, alternating with buttermilk. Begin and end with the dry ingredients. Mix until just combined.
5. Divide the batter evenly among cupcake liners, filling each about 2/3 full. Add a small dollop of the raspberry puree to each and use a toothpick to swirl it gently into the batter.
6. Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool completely.

Prepare the Lemon Buttercream:

1. Beat butter until creamy and smooth. Gradually add powdered sugar, mixing well after each addition.
2. Add lemon juice and zest. Beat until combined. If the frosting is too thick, add heavy cream, one tablespoon at a time, until the desired consistency is reached.

Assemble the Cupcakes:

1. Once the cupcakes are cool, pipe or spread the lemon buttercream frosting on top.
2. Drizzle any leftover raspberry puree on the frosting for decoration, if desired.