In a saucepan over medium heat, combine fresh raspberries, sugar, and lemon juice. Stir until it reaches a boil, then simmer for about 5 minutes. Whisk in cornstarch mixed with water until thickened, then cool completely.

Preheat your oven to 350°F (180°C). Line a cupcake pan with paper liners.

In a mixing bowl, whisk the flour, baking powder, baking soda, and salt. In another bowl, mix sugar and lemon zest. Beat in softened butter, then add eggs and vanilla. Finally, mix in lemon juice and buttermilk.

Fill cupcake liners about two-thirds full and bake for 15-25 minutes or until a toothpick comes out clean. Cool in the pan for 5 minutes before transferring to a wire rack.

Once cooled, use a knife to cut out a small circle from each cupcake center and fill with raspberry jam.

For the buttercream, whisk egg whites and sugar in a heatproof bowl over simmering water until mixture reaches 160°F. Beat until stiff peaks form, then add butter, vanilla, and raspberry jam.

Pipe the buttercream onto filled cupcakes and optionally decorate with fresh raspberries.