Step 1: Bake the Cupcakes

1. Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
2. In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a large bowl, beat butter and sugar until light and fluffy.
4. Add eggs one at a time, then mix in lemon zest, lemon juice, and vanilla extract.
5. Gradually add dry ingredients, alternating with buttermilk, mixing until just combined.
6. Divide batter evenly into cupcake liners, filling each about ⅔ full.
7. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
8. Let cupcakes cool completely before adding filling and frosting.

Step 2: Prepare the Raspberry Filling

1. In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat.
2. Cook until raspberries break down and become syrupy.
3. Stir in the cornstarch mixture and cook until thickened. Remove from heat and let cool.

Step 3: Make the Lemon Buttercream Frosting

1. Beat butter until creamy. Gradually add powdered sugar, mixing until smooth.
2. Add lemon juice, zest, and heavy cream. Beat until light and fluffy.

Step 4: Assemble the Cupcakes

1. Use a small knife or cupcake corer to remove the center of each cooled cupcake.
2. Spoon raspberry filling into the hollowed-out center.
3. Pipe lemon buttercream on top using a star tip.
4. Garnish with a fresh raspberry and enjoy!