Make the blackberry curd. Combine the blackberries with 2 tablespoon lemon juice in a medium saucepot over medium heat. Simmer the berries until they soften, about 5-7 minutes. Remove from heat and puree the berries. Strain the mixture and discard the seeds.
Whisk together the 3 eggs, 1 yolk, and ½ cup sugar until well combined. Slowly add the blackberry puree, whisking constantly. Return the berry mixture to heat in a medium saucepot over low heat. Simmer the curd until thickened, stirring frequently, about 10 minutes. Remove from heat and add the butter a little at a time, stirring until the butter is fully incorporated. Transfer the curd into a bowl and place plastic wrap directly on top of the curd to prevent a skin from forming. Let cool completely.
Preheat oven to 325 degrees F. Line two 9-inch cake pans with parchment paper.
Prepare the lavender lemon chiffon cake. Combine the boiling water with 1 tablespoon lavender and let sit for 10 minutes. Strain the lavender and let cool completely. Grind the remaining 2 tablespoon lavender until finely ground.
Seperate the eggs and place the egg whites in a the bowl of a stand mixer. Add the cream of tartar and whisk the eggs until stiff peaks form.
Meanwhile, sift the ground lavender with the cake flour, sugar, baking powder, and salt into a large bowl. Create a well in center and add the wet ingredients including the oil, egg yolks, vanilla, lemon extract, lemon zest, and lavender infused water. Mix the ingredients until well combined.
Gently fold in the egg whites to the batter until fully incorporated. Pour the batter into the prepared cake pans and bake the cake for 25 minutes or until a toothpick inserted in the center comes out clean. Let cool completely on wire racks.
While the cake cools, make the honey buttercream. Beat the softened butter until light and fluffy, about 3 minutes. Add the honey and beat for another minute. Slowly add the sifted powdered sugar and beat until fully incorporated. Add the milk and beat until well combined.
Cut the cakes in half to form 4 individual layers. Pipe the buttercream around the edge of the first layer and fill the center with the blackberry curd. Stack a layer on top and repeat. Place the last layer on top and spread the frosting on the top and around the edges to cover the cake. Place dollops of the curd on the top of the cake and swirl it in with the buttercream to create a pretty pattern. Top with fresh blackberries and serve.