Finely zest the lemons into a small mixing bowl using a microplane grater.

Juice zested lemons using a citrus juicer, then place the lemon juice, egg yolk and sugar in a heatproof bowl over a pan of simmering water (double boiler) and cook the curd over medium-high heat whisking constantly.

After about 8-10 minutes, the mixture starts to thicken like a custard (at this point the curd will reach around 80C /176F).

Remove it from heat, and pour the cooked curd through a sieve. With a rubber spatula, mix in room temperature butter in small chunks one by one, making sure each chunk is fully incorporated before adding the next. Hand blend, if desired.

Fold the zest in at this point. Pour the curd into a jar and place plastic wrap directly on top so it is touching the top of the curd. (This prevents skin forming on top.) The curd will continue to thicken as it cools in the fridge.

Prepare your cupcake tin by lining it with muffin liners and preheat your oven to 175℃ / 350℉ (no fan).

Massage the lemon zest into the granulated sugar for a minute.

In a large bowl, cream the room-temperature butter and the lemon sugar for a few minutes until light and fluffy.

Then, whip in the room-temperature eggs one by one, and finally, mix in the vanilla extract.

Sift together the flour, baking powder, and salt. This is your dry ingredients. Reduce the mixer speed to low speed and add the dry ingredients in thirds alternating with the room temperature milk. Do not overmix the batter at this point.

Finally, fold in the poppy seed.

In a small bowl, mix vinegar with the baking soda. It will be super bubbly. Then, immediately fold the vinegar mixture into the batter just until it's combined.

Pipe or spoon the batter evenly into the cavities of your cupcake tin, filling each one about ¾ full.

Bake the cupcake for about 20-22 minutes at 175℃ / 350℉ or until a toothpick inserted into the center comes out clean. Do not overbake them, as they can dry out.

Cool the baked cupcakes on a cooling rack. They must be at room temperature before frosting.

Place the mascarpone into a mixing bowl. Add very cold heavy cream to the bowl along with sifted powdered sugar.

Whip it up with an electric hand mixer. When the frosting reaches a fluffy (after 2-3 minutes), pipeable consistency, stop right there and do not overwhip.

Once the cupcakes cooled to room temperature and the lemon curd sets, make a small hole on each of the cupcake and fill it with the lemon curd.

Move the frosting into a piping bag, and pipe it on top of the cupcakes then fill them with more lemon curd.

Dust with some more poppy seed and decorate with edible flowers, if desired.

Serve the cupcakes fresh. Any leftovers can be stored covered and refrigerated for 1-2 days. Bring them back to room temperature before serving for the frosting to soften.