For the cupcakes: Preheat the oven to 325 degrees F. Line a cupcake or muffin pan
with 24 regular-size cupcake liners.
Place the oil, buttermilk, eggs
, vinegar, vanilla, and sugar in the bowl of an electric stand mixer
and beat with the paddle attachment until thoroughly mixed. Sift
the flour, baking soda, salt, and cocoa powder
together and slowly add to the mixer on low speed. Scrape down the bowl thoroughly, as needed. Add the food colorings in a steady stream until desired shade is achieved; it should appear bluish-purple. Beat for another 3 minutes. Scrape the bowl one last time and mix for 30 seconds by hand.
Fill the cupcake liners two-thirds full with batter
and bake until they form a semi-hard crust
on top, about 25 minutes. Immediately remove the cupcakes from their pan and place on a cooling rack
For the blackberry curd filling: Bring the blackberries and water to a boil in a medium-size saucepan. Turn down the heat to low and simmer
until the blackberries are soft and separate easily, about 5 minutes. Transfer the mixture to a sieve
and pass through using a spoon to help mash
the blackberries. Discard the pulp and reserve the juice. Place the flour and sugar
in a clean saucepan
and slowly pour in the blackberry juice, reserving 2 tablespoons. Whisk
constantly. When thoroughly combined, add the whole eggs and egg yolks, whisking and cooking until the mixture thickens. When the curd has formed, after about 7 minutes, remove from the heat and stir in the butter. Cover tightly and place in the refrigerator until fully chilled.
For the cream cheese
frosting: Beat the cream cheese and butter together in the bowl of an electric stand mixer until pale and creamy. Add the blackberry juice and lemon zest and continue to beat until the color is even. (The lemon zest
has a tendency to adhere to the beater blade. When removing from the mixer bowl, be sure to scrape this off and blend
back into the frosting
by hand.) Reduce the speed to low and slowly begin adding the confectioners' sugar. Keep adding until the frosting has a medium-stiff consistency. You may not require the full 5 cups sugar. Beat on high speed for a further 2 minutes, scraping down the bowl as needed.
To assemble: Remove the center of each cupcake using an apple corer
. Fill the cavity with the blackberry curd
until it is level with the top of the cake. Pipe the frosting on top using a plain pastry
tip. Crumble one of the cakes
into small pieces and scatter a few crumbles on top of each cake so that it adheres to the frosting. Garnish
with a whole fresh blackberry, if desired.