Butter two 10-inch cake pans and dust them with flour. Line them with parchment rounds and set aside.
In a large bowl, whisk together the cake flour, baking powder and salt. In the bowl of a stand mixer beat the butter and 1 ⅔ cups sugar until light and fluffy. Add vanilla and lemon zest.
Add the flour mixture and milk in 3 parts, alternating additions of each and beginning and ending with flour. Beat on low until mixture is uniform, add lemon juice and set aside.
In the bowl of a stand mixer, beat the egg whites and remaining cup of sugar until stiff glossy peaks form. Fold egg white mixture into the cake batter. Divide between prepared tins and bake for about 30 to 40 minutes, until sides of the cake begin to pull away from the tin.
With a stand mixer, beat butter until light and fluffy.
Gradually add in powdered sugar one cup at a time, scraping down the bowl as necessary.
Finally, add the milk slowly until desired consistency is reached.
Tint the buttercream with food coloring as desired and mix well. Set aside.
Line two large baking sheets with parchment paper.
Melt all candy melts and mix colors if desired until at least 4 tones are achieved.
Drop quarter-sized drops of melted candy melts in two lines on your prepared sheet tray, 4 inches apart. Before the candy melts set, pull the brush through the drop and create a 4 to 5 ½ inch brush stroke with each candy melt drop. Set brushstrokes aside and allow the candy melts to set at room temperature.
When the candy melts are set, dip your paintbrush into the vanilla vodka and then into a bit of your desired luster dust. Brush the metallic mixtures onto some of the brushstrokes, experimenting with color combinations.
Assemble the cooled cake layers by crumb coating the cake, placing fresh blackberries in the center of the cake. Chill then finish the final buttercream layer.
While the buttercream is still wet on the cake, decorate it with the candy melts. Drizzle some candy melts over the cake and add sprinkles as desired over the cake. Apply the brushstroke candy melts around the side of the cake and around the top.