Step 1: In a saucepan, combine fresh raspberries, sugar, and lemon juice over medium heat. Stir until it reaches a boil. Allow to simmer until thickened, about 5-7 minutes. Remove from heat and let cool.

Step 2: Preheat your oven to 350°F (175°C). In a mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and sea salt. In another bowl, cream together granulated sugar and lemon zest with the butter. Incorporate eggs, vanilla, lemon juice, and buttermilk until smooth.

Step 3: Divide the batter evenly among lined cupcake cups, filling each about three-quarters full. Bake for 15-20 minutes or until a toothpick comes out clean. Let cool completely.

Step 4: Cut the tops off each cupcake to create a well. Fill each well with raspberry jam and replace the tops gently.

Step 5: Set up a double boiler and combine egg whites and sugar, whisking continuously until warm and sugar dissolves, about 4-6 minutes. Whip until stiff peaks form.

Step 6: Gradually add butter, mixing on low speed until incorporated. Whip until fluffy, then stir in vanilla, salt, and jam. Mix until smooth.

Step 7: Frost each cupcake with Swiss meringue buttercream. Optionally, garnish with fresh raspberries.