1-Preparing the Lemon Curd: Make the lemon curd first to allow it to cool and thicken. Separate yolks from egg whites before starting. Combine 1 cup granulated sugar and 2 tablespoons lemon zest; massage the zest into the sugar. Whisk in 6 large egg yolks and 1/2 cup lemon juice. Heat the mixture on medium-low while whisking until thickened and bubbling. Remove from heat, stir in 1/2 cup cold unsalted butter (cut into tablespoon pieces) until melted. Strain the lemon curd for a smooth texture, cover, and cool.

2-Preparing the Lavender Milk: Prepare lavender milk by heating 1 cup whole milk to a simmer, then remove from heat and steep 3 tablespoons dried lavender for 15 minutes. Strain and cool before use.

3-Making the Lemon Lavender Cake: Preheat the oven to 350°F (175°C). Grease and line three 8-inch or four 7-inch cake pans. Mix 3 cups granulated sugar and 2 tablespoons lemon zest, then combine the dry ingredients: 3 cups cake flour, 2 1/2 teaspoons baking powder, and 1 teaspoon fine salt. Cream in 1 cup (2 sticks) unsalted butter (room temperature) until the mixture looks like moist sand. Add 1 cup carton egg whites or 7 large egg whites (room temperature), and mix until incorporated. Mix in 1 cup full-fat sour cream (room temperature), 1/2 cup lavender milk (prepared as above), 2 tablespoons vegetable or canola oil, and 1 teaspoon vanilla extract or vanilla bean paste. Beat at medium speed briefly to combine. Divide the batter, coloring half purple and half yellow if desired using purple and yellow gel food coloring (optional). Alternate spoonfuls in the pans and swirl. Bake for 33-36 minutes or until a toothpick inserted comes out with moist crumbs. Cool in the pans for 10 minutes, then transfer to racks. Level the tops if needed. Freeze the layers if not assembling immediately.

4-Creating the Lavender Swiss Meringue Buttercream: Make lavender Swiss meringue buttercream by whisking 8 large egg whites (room temperature) and 2 1/2 cups granulated sugar over simmering water until the temperature reaches 160°F and the sugar is dissolved. Whip in a stand mixer until stiff peaks form. Add 2 cups (4 sticks) unsalted butter (room temperature) gradually, then 1/4 cup lavender milk (room temperature), 2 teaspoons vanilla extract or vanilla bean paste, and 1/2 teaspoon fine salt, mixing until smooth. Color the frosting after cake assembly.

5-Assembling the Cake: Assemble the cake by stacking the layers on a board or plate using a dab of frosting to secure the base. Spread a thin layer of buttercream on each layer, pipe a ring of frosting around the edges, and fill the center with lemon curd (approximately 1/2 cup per layer). Flip the top cake layer upside down for sharper edges. Freeze for 5-10 minutes, then apply a crumb coat and chill again until firm. Add the final frosting layer and texture with a spatula. Color reserved frosting yellow and purple using purple and yellow gel food coloring, and blend colorful swipes onto the cake with a spatula for decoration.