In a saucepan over medium heat, combine blueberries, sugar, lemon zest, and lemon juice. Cook for about 8–10 minutes until the berries release their juices.

Stir together cornstarch and warm water to make a slurry. Add to the blueberry mixture and cook for another 2–3 minutes until thick and glossy. Remove from heat, stir in vanilla, and let cool completely.

Simmer 3 cups of blueberries in a saucepan over medium heat until thick and jam-like, about 10–15 minutes. Mash slightly and let cool.

Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans.

In a bowl, whisk together cake flour, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, beat softened butter and granulated sugar until light and fluffy, about 3–4 minutes.

Mix in vegetable oil and sour cream until smooth. Add blueberry reduction and the vanilla extract. Stir until combined.

Beat in eggs and egg white one at a time until fully incorporated.

Gradually add dry ingredients in three additions, alternating with milk, mixing just until combined. Don’t overmix.

Divide batter evenly between prepared pans. Bake for 25–30 minutes until centers spring back and a toothpick comes out with a few moist crumbs.

Let the cakes cool in pans for 10 minutes, then transfer to racks to cool completely.

Beat softened butter until creamy. Gradually add powdered sugar, mixing until fluffy.

Add blueberry filling, crushed freeze-dried blueberries, vanilla, salt, and cream. Beat until smooth. Adjust sweetness by adding more powdered sugar if desired.

Place the first layer on a serving plate, spread a thin layer of frosting, and pipe a border around the edge.

Spoon blueberry filling inside the border. Repeat with the second layer.

Place the final layer on top and frost the entire cake with the remaining buttercream.

Chill for about 20–30 minutes before slicing.