Heat raspberries, sugar, and lemon juice in a saucepan over medium heat for 10-12 minutes until it boils. Mash the berries, then whisk cornstarch and water together, and stir it into the jam. Cook until thickened and cool before use.

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

In a bowl, whisk together the flour, baking powder, baking soda, and salt.

In a stand mixer, mix the sugar with the lemon zest, then add the flour mixture and softened butter. Mix until combined.

In another bowl, whisk the eggs and vanilla together, and then add to the batter. Mix for 2-3 minutes until fluffy.

Slowly mix in the lemon juice and buttermilk. Fill cupcake liners about 3/4 full.

Bake for 15-18 minutes for regular cupcakes or 20-25 minutes for larger ones, until a toothpick comes out clean. Let them cool on a wire rack.

Once cooled, fill the cupcakes by removing the center and adding raspberry jam.

Create a double boiler, and whisk the egg whites and sugar until the mixture reaches 160°F (72°C).

Whip this until it is glossy and has stiff peaks.

Gradually add softened butter to the meringue, mixing until smooth.

Add vanilla and raspberry jam, then mix to combine.

Transfer the buttercream to a piping bag and pipe it onto the cooled cupcakes.

Finally, top with fresh raspberries.