Preheat the oven to 350F. Butter and flour three 6-inch cake pans. Add on damp baking strips. Zest and juice a lemon.
In a stand mixer, sift in the flour, baking powder, salt and sugar. Give it a whisk and set aside.
In a large bowl, add egg whites, milk, lemon juice, lemon zest and sour cream. Whisk together.
Add a paddle attachment to the stand mixer. Add in room temperature butter in cubes or pieces.
Switch to a whisk attachment. On medium speed, beat for ½ of the wet mixture into the crumby dry mixture. Scrape the bowl down and add the remaining wet mixture. Beat until combined.
Pour the batter into the pans. I use a kitchen scale for more precision.
Bake for 33 - 36 minutes and/or until a toothpick inserted comes out clean.
Let the cake layers cool in their pans for about 5 minutes, them dump them out onto a wire rack to cool to room temperature.
For the Blackberry Curd:
Cut cold butter into 1 inch cubes.
Strain the egg yolks through a sieve. Beat with a french whisk.
While mixing, add in the sugar. It’ll turn into a light lemon color.
Add a clamshell of blackberries to a plastic ziplock bag.
Using a rolling pin, mash the berries into a pulpy consistency.
Use a muddler and a sieve to strain any juice through.
Measure out the juice to be about ⅓ cup or 80ml juice.
Add the blackberry juice to the mixture.
Transfer the bowl to a medium low heat while whisking constantly.
Once it hits 170F, it will change consistency. This can be tested by dipping the back of a wooden spoon and running your finger through.
Add in the cubed butter until combined.
Pour into a bowl, cover with plastic and transfer to the fridge to set for a few hours or even overnight.
When set, transfer to a piping bag for assembly. Snip off the tip.
For the Frosting:
In a standing mixer fitted with a paddle attachment, cream the butter.
Sift in the confectioners sugar. Mix on low. Add a dash of orange blossom water or your choice of flavor.
Add in the room temperature mascarpone cheese. Mix on low for about 5 seconds. If you mix too long, the frosting will become grainy.
Transfer to a piping bag. Snip off the tip.
For the Assembly:
Use a serrated knife to cut 2 layers in half. This step is optional. I wanted my layers to be thinner.
Pipe the mascarpone frosting first. Add blackberries. Drizzle some blackberry curd over the berries. Add the next layer and repeat the process.
Cover the cake in a layer of mascarpone frosting. Use a bench scraper and offset spatula to even of the first layer. Transfer to the fridge to chill. Depending on the climate and weather you’re decorating in, you may need to chill the cake between each of the assembly.
Add a layer of curd on the top of the cake. Use an offset spatula to help your drips fall down the sides.
Add a crown of blackberries to the top of the cake. I added a few a few buds of Queen Anne’s Lace throughout the berries. This flower is edible but not tasty so remove before eating.
Video
Notes
Making components ahead.
You can make the curd a few days in advance and refrigerate it. Cake layers can be made ahead of time, wrapped and left at room temperature overnight or refrigerated for a few days. The buttercream should be made the day of assembly.
What if you can’t find Mascarpone anywhere?!?
A while back I was making a cake with a mascarpone buttercream and realized mine had expired! I didn’t have time to go to the market so I followed Nigella’s advice for a reasonable facsimile and mixed the following ingredients together until combined. Make sure they are all at room temperature!