Prepare the Raspberry Swirl: In a small saucepan, combine raspberries, sugar, and lemon juice. Cook over medium heat for 5-7 minutes, mashing the raspberries to release their juice. Simmer until thickened, then strain to remove seeds. Let cool.

1. Make the Cupcakes: Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners. In a bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
2. In another bowl, cream the butter and sugar until light and fluffy. Add the eggs, one at a time, beating well after each. Stir in vanilla extract.
3. Alternately add the dry ingredients and buttermilk to the butter mixture, starting and ending with the dry ingredients. Mix until just combined.
4. Spoon the batter into the muffin pan, filling each cup about 2/3 full. Drop a teaspoon of the raspberry puree onto each cupcake and use a toothpick to swirl the puree into the batter.
5. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool completely on a wire rack.
6. Make the Frosting: Beat the softened butter until creamy, then gradually add powdered sugar, vanilla, cream, and salt. Beat until smooth and fluffy.
7. Once the cupcakes are completely cool, frost them with the buttercream frosting. Top with additional raspberry puree for extra swirl effect.