Combine fresh lemon juice, zest, sugar, eggs, and butter in a heatproof bowl over simmering water or gently in a saucepan. Stir constantly until thickened to a custard-like consistency, then strain for smoothness and chill until firm.

Pro Tip: The curd should coat the back of a spoon and be glossy.

Whisk egg whites and sugar over a double boiler until warm and sugar dissolves. Whip to glossy, medium-stiff peaks.

Gradually beat in room-temperature butter until smooth, then fold in lemon zest and a splash of lemon juice.

Pro Tip: It should be smooth and pipeable, not soupy.

Whisk dry ingredients together. Cream butter with sugar until light, then alternate adding dry ingredients and milk.

Beat in eggs, then fold in poppy seeds and lemon zest. Spoon batter into lined muffin tins and bake until a toothpick shows moist crumbs.

Pro Tip: Tops should be pale gold with spring-back.

Core each cooled cupcake and pipe chilled lemon curd into the cavity until just level with the top.

Pro Tip: Avoid overfilling to prevent leaking.

Fit a piping bag with your chosen tip, fill with buttercream, and pipe a swirl over each cupcake.

Pro Tip: Should hold its shape and show ridges.

Store cupcakes in an airtight container in the refrigerator for up to 3 days.

Freeze unfrosted cupcakes or freeze assembled cupcakes on a tray and then transfer to a freezer-safe container.