In a medium saucepan, combine fresh raspberries, sugar, and a splash of lemon juice over medium heat. Stir continuously until the mixture begins to bubble, about 5-7 minutes. Cook until thickened, resembling a syrupy consistency, then remove from heat and let it cool completely.

In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, and fine sea salt. In another bowl, combine granulated sugar with fresh lemon zest, mixing until fragrant. Add softened unsalted butter to the sugar mixture and beat until creamy, then incorporate eggs, vanilla bean paste, lemon juice, and buttermilk, mixing until the batter is smooth.

Line a cupcake tin with paper liners and preheat your oven to 350°F (175°C). Evenly distribute the cupcake batter into the liners, filling each about ¾ full. Bake for 15-25 minutes or until the tops are golden brown and a toothpick inserted in the center comes out clean. Once baked, transfer them to a wire rack to cool completely before filling.

Once the cupcakes have cooled, use a sharp knife to carefully cut off the tops, creating a small hole in each cupcake. Scoop a bit of the cupcake out using a small spoon or a cupcake corer and fill each hole generously with the cooled raspberry jam.

In a heatproof bowl over a double boiler, whisk the egg whites and additional granulated sugar until the mixture reaches 160°F (71°C). Remove from heat and using an electric mixer, whip the whites until glossy stiff peaks form, around 10 minutes. Gradually add softened butter, vanilla bean paste, and a swirl of raspberry jam until the frosting is silky smooth and fluffy.

Transfer the raspberry Swiss meringue buttercream to a piping bag fitted with your favorite nozzle. Pipe the frosting onto each cupcake in a swirl pattern, making them look absolutely delectable. Finally, garnish with fresh raspberries.