Preheat oven to 350º F. Place baking cups on a baking sheet. If you are using paper cups like I did, no need to spray of grease. If using liners, place then in the wells of a muffin tin.
Add the lemon juice to the milk and set aside.
Cream butter and sugar using the paddle attachment of stand mixer.
Gradually beat in sugar until light and fluffy.
Lightly whisk the eggs with the vanilla beans, lemon zest and ginger and add to the creamed butter. Mix on low until fully combined.
Blend flour, salt and baking powder and add to the butter mixture in three additions, alternating with the milk mixture. Beat until smooth.
Remove about a half cup of the batter and add to a small bowl. Sift in the blueberry powder and mix with a spatula until well blended.
Divide batter evenly among the cups - I weigh mine to be certain. Add a dollop of the blueberry batter and use a toothpick or small knife to swirl into the plain batter.
Bake for 20-25 minutes. Test with a cake tester to toothpick to be sure. Cool completely before decorating.
To make the buttercream:
In a medium saucepan, add the water and the first measure of sugar and cook without stirring until the mixture reaches (116ºC/240ºF), or what is labelled as 'soft ball' on your thermometer.
While the sugar is cooking, add the egg whites to the bowl of a stand mixer fitted with the whisk attachment and set aside.
Once the sugar syrup reaches 110C/230F, start whipping the eggs on medium speed. When the egg whites begin to appear foam, sprinkle in the second measure of sugar while whipping.
The egg whites should be at soft peaks by the time the sugar reaches 116C/240F/soft ball.
With the egg whites whipping on medium speed, rest the lip of the pot on the lip of the mixing bowl and pour the syrup into the whipping egg whites in a slow, steady stream down the side of of the bowl.
Continue to whip until the mixture has cooled to room temperature and the meringue is full and billowy.
On medium speed, add the butter in small chunks and continuing to whip until the smooth and light. Add the lemon curd and whip on low until fully incorporated.
Assembly:
Top each cupcake with a thin layer of buttercream - this ensures the top of the cupcake is covered and the piped flowers have a foundation for attaching.
Spread a generous layer of the lemon curd buttercream over a sheet of plastic wrap, then spoon the blueberry buttercream in the shape of a log down the middle of the lemon buttercream.
Roll up the plastic wrap and place in a piping bag fitted with the nozzle of choice. Pipe flowers, starting from the middle.