In the bowl of an electric mixer fitted with the paddle attachment, beat the butter at medium low speed until smooth. Reduce the speed to low, add the powdered sugar and the cooled blackberry mixture, and continue beating until the mixture is just incorporated. Increase the speed to medium and beat until the buttercream becomes light and fluffy, about 3 to 5 minutes.

Preheat the oven to 350 degrees Fahrenheit and grease two 9-inch cake pans, lining the bottom of each one with a circle of parchment paper. Set them aside.

In a medium bowl, mix together the flour, baking powder, and salt and set aside.

In the bowl of an electric stand mixer fitted with the paddle attachment, beat together the butter and sugar at medium low speed until fluffy and smooth. Add the egg whites and egg, one at a time, mixing well after each addition.  Add the honey, vanilla extract, lemon juice, and lemon zest and mix until combined.

Mixing at low speed, add half of the flour mixture to the butter mixture, then add half the milk. Repeat once more so that all of the flour mixture and milk are now in the batter. Mix until the batter just smooths out. Evenly distribute the batter between the two cake pans and place in the oven. Bake until a toothpick inserted into the center of each one comes out clean, about 40 to 45 minutes.

Remove the cakes from the oven and allow to cool for 10 minutes before inverting the cakes onto a wire rack to remove them from the pans and allowing them to cool completely. Peel off and discard the parchment paper.

Once cooled, use a long bread knife to cut the cakes in half horizontally so that you now have 4 layers instead of 2. Layer and frost the cake with the blackberry buttercream. Garnish with the fresh blackberries and serve.

NOTE: The hydrangea flowers on top of the cake pictured are for decoration ONLY. They are *NOT* edible flowers.