Place poppyseeds in a cup or a bowl with water and soak them for about 25- 30 minutes.
Heat the oven to 185C (170C fan) and line the muffin tin with 12 cupcake liners. In a bowl of a standing or hand-held mixer, cream together butter and sugar for about couple of minutes until pale, light and fluffy. Add eggs, one at a time, mixing well after each addition. Add flour, baking powder and lemon zest and stir until just combined.
Drain the poppyseeds and add them to the batter along with lemon juice and vanilla extract. Stir again until you have smooth batter.
Divide the batter between cupcake liners, filling about 2/3 of each case. Place in the oven and bake for about 25 minutes or until risen and springy to the touch. Remove from the oven and place on a wire rack to cool completely.
To make Swiss meringue buttercream:
In a large bowl, or a bowl of your electric mixer, add the egg whites and caster sugar and mix by hand briefly, until they are just combined.
Create a double boiler by placing your bowl over a pan of simmering water. Heat the egg mixture, stirring often, until the temperature on the sugar thermometer reaches 70 C. The mixture should be very hot to the touch and the sugar should have dissolved completely.
Return the bowl with egg whites and sugar to the mixer, and using a whisk attachment, beat the mixture on high speed for about 8 minutes, until it's shiny and medium- soft peaks have formed. The meringue should be room temperature when this is finished. Stop the mixer and swap for the paddle attachment.
With mixer running on low, start adding softened butter, 2- 3 tbsp at a time. Once all the butter have been incorporated, add the vanilla and poppyseeds (if using). Turn the mixer to medium speed and and mix until silky smooth.
To assemble the cupcakes:
Using a teaspoon or the back of your piping nozzle, create small holes on top of each cupcake and fill each cupcake with about 1/2 tsp of lemon curd.
Transfer the buttercream into a piping bag fitted with the nozzle (I've used Wilton 1M) and pipe the buttercream on top of each cupcake. Sprinkle with extra poppyseeds if you would like and enjoy!