Basically sunshine in cupcake form, with some poppy seeds thrown into the mix – that’s probably the best description of these lemon poppy seed cupcakes. Each of the components (the soft and delicate cupcakes, the tangy lemon curd, the fluffy Swiss meringue buttercream) are amazing on their own. But together, they’re simply to die for.
Pre-heat the oven to 355 ºF (180 ºC) and line a 12-hole cupcake/muffin baking tin with cupcake liners.
In a small bowl, whisk together the milk, eggs and lemon juice until homogeneous.
In a separate larger bowl, sift together the gluten free flour, xanthan gum, baking powder, sugar and salt. Add the lemon zest and poppy seeds and mix briefly.
Add the milk+egg mixture and whisk until you get a smooth cupcake batter.
Evenly distribute the cupcake batter among the 12 cupcake cases – they should be each about 3/4 filled.
Bake in the pre-heated oven at 355 ºF (180 ºC) for 18 - 20 minutes or until an inserted toothpick comes out clean and the cupcakes are slightly domed and golden brown on top.
Allow to cool.
In a medium-sized saucepan, combine all lemon curd ingredients except the butter. Whisk or stir briefly until the eggs are evenly incorporated.
Cook the lemon curd on medium-high heat until it starts thickening – stir constantly but do not whisk. Whisking will create a foam (from the egg yolks and the sugar), which will make your lemon curd frothy and not at all "lemon curdy".
After about 8 - 10 minutes, the lemon curd should have thickened significantly. Remove from heat and stir in the butter, 1 tbsp at a time, until all butter has melted and has been completely incorporated.
Allow the lemon curd to cool, stirring occasionally to prevent "skin" formation on top.
Mix the egg whites, sugar and cream of tartar in a heat-proof bowl, and set the bowl above a pot of simmering water.
Heat the meringue mixture with constant stirring until it reaches 150 ºF (65 ºC) and the sugar has melted.
Transfer the heated meringue mixture into a stand mixer with a whisk attachment (or use a hand mixer with the double beater attachment) and whisk the mixture for 5 - 7 minutes, until it reaches room temperature and forms glossy peaks. Do not over-whisk as you can lose some of the fluffiness.
(NOTE: The meringue must be at room temperature before you start adding the butter – otherwise, you'll get a soup and not a deliciously fluffy buttercream!)
Add the butter, 1 - 2 tbsp at a time, while constantly whisking/beating at medium speed. If using a stand mixer, I prefer to switch to a paddle attachment for this step.
Continue until you've used up all the butter.
The buttercream will probably look like it's split at some stage (or it might become very runny, almost "soupy") – don't worry, that's normal. Just continue adding the butter and whisking/beating. The buttercream will eventually become velvety smooth. Be careful not to over-beat it, though – you will lose the air (and fluffiness) from the original meringue!
Add the salt and poppy seeds, and whisk/beat briefly until evenly incorporated.
The lemon poppy seed cupcakes keep well in a closed container in a cool dry place for 3 - 4 days.