To make the frosting, in a stand mixer attached with the whisk attachment, whip the heavy cream and powdered sugar on medium speed until the cream is thick. Add in the mascarpone and whip on medium-high speed until firm peaks form. Be careful not to over whip or the frosting will turn into butter. Transfer the frosting to an airtight container and refrigerate until ready to use.
To make the Earl grey sugar, process tea leaves and 1/2 cup of the sugar until leaves are finely ground.
Sift flour, baking powder, and salt into a large bowl.
Preheat the oven to 350 degrees F and place a rack in the center of the oven.
Line 2 8-inch cake pans with parchment paper and grease the bottom with olive oil. Set aside.
Beat egg whites in a large bowl with an electric mixer with the whisk attachment on medium-low speed until frothy, 2 minutes. Add cream of tartar and beat on medium-high speed until soft peaks form, about 3 minutes.
Gradually add remaining sugar, one tablespoon at a time, waiting for 10 seconds before adding another tablespoon of sugar. Continue beating until mixture is thick and glossy, about 5 minutes. The egg whites will look like shaving cream. Transfer the whipped egg whites to a clean bowl.
In the same bowl that you used to whip the egg whites, add Earl Grey sugar and egg yolks. Beat for 5 minutes or until thick and pale, and doubled in volume. Gradually add milk and olive oil, beating until well combined.
Add in the sifted dry ingredients and beat until well combined. Gently fold the egg white mixture into the cake batter in four batches with a rubber spatula, until just combined.
Divide the batter evenly between the prepared cake pans. Scatter ¾ cup of blackberries over the batter. Smooth the top with an offset spatula.
Place cake pans on the middle rack and bake for 30-35 minutes or until a skewer inserted into the center comes out clean.
Remove the cakes from the oven and let them cool in the pans on a wire rack for 5 minutes.
After 5 minutes, immediately turn the pans upside-down on the cooling rack and leave to cool completely in this position.
When the cakes are completely cooled, turn them right side up and carefully run a small knife around the edge of the cake pan to help release the cake.
Peel off the parchment paper. If the cakes dome a bit, use a serrated knife to trim it so the cake is level.
Using a serrated knife, split the cake in half horizontally. You should get 4 layers.
Place one cake on a flat serving plate. Spoon 1 ½ cups of mascarpone frosting on the cake and spread it evenly across the top.
Place the second cake layer on the frosting and press down slightly to even out the cake. Repeat with the remaining layers.
Using an offset spatula, spread the sides of the cake with remaining frosting.
Use a bench scraper to smooth the sides of the cake to achieve a naked look.
Decorate with flowers and blackberries.
Refrigerate for at least 2 hours before serving and up to 3 days.
When ready to serve, slice the cake into 8 pieces, transfer to plates, and top with fresh blackberries.