Marble cake:

1. Preheat the oven to 350°F (175°C). Butter two 4-inch round cake pans and line the bottoms with parchment rounds. Dust the sides with flour and tap out any excess. Line 3 cups of a muffin tin with cupcake liners.
2. In a large bowl, whisk together the flour, baking powder, baking soda, salt, and vanilla bean seeds (or vanilla extract) until combined.
3. In the bowl of a stand mixer fitted with the whisk attachment (or using a large bowl and handheld mixer), beat the egg whites on medium-high speed. After a few seconds, slowly stream in the sugar while continuing to beat. Beat for 3–4 minutes until white and fluffy.
4. Reduce the mixer to medium speed. Slowly pour in the melted butter while beating. Beat until just incorporated, scraping down the bowl as needed.
5. With the mixer on low, alternate adding the flour mixture and buttermilk in three additions, beginning and ending with the flour (flour ? buttermilk ? flour ? buttermilk ? flour). Mix just until combined after each addition; do not overmix.
6. Remove half the batter to a separate bowl. Add the 1/4 cup buttermilk and cocoa powder for the chocolate marble to the remaining batter in the mixer and mix on the lowest speed until mostly incorporated; finish folding by hand.
7. Alternate spoonfuls of the white and chocolate batters into the prepared cake pans and cupcake liners, filling each about 3/4 full. Run a knife or small spatula through each once or twice to create a marble pattern.
8. Bake until a toothpick inserted in the center comes out clean or with a few moist crumbs — about 20 minutes for cupcakes and 35–40 minutes for the cakes. Cool cupcakes in the pan for 5 minutes, then transfer to a wire rack. Let cakes cool completely in pans on a wire rack before removing.
9. Once the cakes are completely cool, wrap tightly in plastic wrap and freeze for 30 minutes before frosting (this firms the crumb and makes layering easier).

Blackberry cream cheese frosting:

1. In the bowl of a stand mixer fitted with the whisk attachment, beat the butter and cream cheese together on medium-high speed until smooth and creamy.
2. Add the strained blackberry juice and vanilla extract and beat until incorporated.
3. With the mixer on low, gradually add the confectioners’ sugar, scraping down the bowl as needed. Once all the sugar is added, increase speed to medium-high and beat until smooth, fluffy, and creamy. Beat in the pinch of salt.
4. If using gel color, add a small amount to achieve a light lavender hue and fold in by hand until evenly tinted.

Assembling the cake:

1. Remove the cakes from the freezer. Use a large serrated knife to trim the domed tops level. Slice each cake in half horizontally to form four layers total.
2. Set one layer on a cardboard cake round on a rotating cake stand. Spread a layer of frosting over the top with an offset spatula. Stack the next two layers, frosting between each. Add the final layer on top and cover the entire cake with a thin crumb coat of frosting. Refrigerate for 15 minutes.
3. Divide the remaining frosting into three bowls. Leave one portion plain (lightest), and tint the other two increasingly darker shades of lavender with gel color.
4. Remove the cake from the refrigerator. Apply the lightest frosting across the top and push the excess over the edges. Apply the darkest color to the bottom third of the cake, the medium color to the middle third, and smooth all three with an offset spatula to blend them into an ombré effect without fully mixing the colors together.
5. Wipe any excess from the top perimeter so the edges are clean and smooth.
6. Decorate the top with fresh blackberries and mint, and sprinkle with sparkling sugar.

Cupcakes:

1. Scoop a generous mound of frosting onto each cupcake and spread to cover, keeping the frosting slightly domed in the center. Smooth the sides into a neat cone shape with the offset spatula.
2. Garnish each cupcake with a fresh blackberry, a small mint sprig, and sparkling sugar.