Preheat the oven to 350°
Lightly grease two 9” round cake pans with butter and dust with a little flour, shaking off the excess.
Put the blackberries and water in a small saucepan, cover and place over medium-low heat and cook for about 5 – 7 minutes, until they fall apart. Squish them completely with a potato masher or puree it in a mini food processor, and strain through sieve—you should have about ½ cup. Return the juice to the pan and reduce over low down to ¼ cup. Set aside to cool.
Sift the flour, cornstarch, baking powder, cream of tartar and salt together into a large bowl and set aside, too.
Meanwhile, cream the butter and sugar in a stand-up mixer with the paddle attachment on medium speed for 3 minutes, until it’s light, pale and fluffy.
Crack the eggs into a liquid measuring cup and turn the mixer on medium-low. Add the eggs one at a time and let them fully incorporate and emulsify before adding the next. Shut the mixer off immediately once the final egg is mixed in, and scrape the bottom and sides of the bowl with a rubber spatula.
Mix the food coloring into the blackberry puree until it breaks up completely and dissolves. Beat in the milk and extracts.
Turn the mixer back on low and add about a third of the dry ingredients and let them incorporate before pouring in about a third of the wet ingredients. Do this twice more, ending with the wet. Scrape the bowl and mix just until everything is combined. Do not over mix.
Turn out the batter into the prepared cake pans and bake for 30 minutes in your preheated oven. The cakes will be set and almost firm to the touch, but not tough, and will have begun to pull away from the sides of the pan ever so slightly—this is not a sign of over-baking here.
Let the cakes cool in their tins for about 10 minutes before inverting them onto a greased wire cooling wrack until completely cooled.
For the Lime Buttercream
Combine the sugar and corn syrup in a saucepan (about a quart in volume) and place over medium heat. Dip in a candy thermometer and bring it to a boil, keeping it there until it reaches 240°.
Zest the limes over a ceramic plate (a wood cutting board would absorb much of the flavor) and set aside.
Wipe down the inside of the standup mixer’s bowl and whisk attachment with a little lime juice and paper towel to ensure they’re both free of grease. Add the egg whites and cream of tartar and beat on high speed for about 3 minutes, until big full peaks form (but without becoming dry).
The syrup should be just under 240°; add in the lime zest and stir once or twice just to combine a little. Once it reaches 240° turn off the heat and immediately pour it into the egg whites. Pour it in a slow, even stream in between the inside of the bowl and the whisk, being extremely careful not to get any on yourself or the whisk (it can fling hot syrup everywhere).
Keep the mixer on high, whisking until the whites and bowl are completely cooled to at least 70 - 75°. This can take about 15 minutes, more or less, depending on the temperature of your kitchen.
Once it’s completely cooled turn the mixer on medium and add the soft butter, about a third of a stick at a time, letting each piece incorporate almost completely before adding the next.
Mix together the lime juice and food dye until there are no clumps and, once all of the butter has been beaten in, pour in the brilliant green juice. Beat until the buttercream is electrified by neon green.
For the Blackberry Drip:
For the blackberry drip, boil, puree, strain and reduce the blackberries the same way as before until you have ¼ cup.
Sift together the powdered sugar and meringue powder into a mixing bowl.
Stir the food coloring into the puree and pour it all into the sugar-meringue bowl with a spritz of lime. Whisk until combined. The drip should be thin enough to spread easily and drip down the sides of the cake, but thick enough not to run off the cake. Add water, as needed, to thin it out drop by drop.
Put a small spoonful of the frosting in the center of a cake stand and set the first layer of cake on it, flat side facing up. Spread about a cup or so of frosting on the top and then set the other layer on top of that, again with the flat side facing up.
Coat the top and the sides in a layer of frosting to crumb coat, scraping off the access buttercream. Stick the cake in the fridge for a few hours to chill the frosting (it will make it easier to apply the final layer of buttercream).
Once the crumb coat is firm, frost it with the remaining frosting (though you may not want to use all of it). Chill it again for a little bit before pouring the blackberry drip over it (once it hits the cold icing it firms up much quicker). Serve it immediately or give the drip a chance to set up.
The blackberry extract that I use is by LorAnn Oils from King Arthur Flour. It tastes just like fresh blackberries in minimal amounts, but be conservative when using it; it can go from fresh berries to fake candy flavor with just a drop.
Depending on how much of a drip you want on the cake you may not need it all (I usually end up with about a quarter of it left over).