Prepare Blueberry Jam: Combine blueberries, lemon juice, vanilla, sugar, and salt in a medium saucepan. Cook over medium heat for 20-30 minutes, stirring frequently. Mash blueberries if a smoother consistency is desired. Cool in the refrigerator.

Prepare Cake Pans: Preheat oven to 350°F. Grease two 8-inch cake pans and line the bottoms with parchment circles.

Make Cake Batter: Whisk dry ingredients (flour, salt, baking powder) and set aside. Beat eggs and sugar on medium speed for 1 minute, then increase to high speed and beat for 7 minutes until light and fluffy. In a saucepan, heat milk and butter until melted (do not simmer). Whisk vanilla and oil into the milk mixture. Add the dry flour mixture to the eggs in three parts, mixing until almost combined. Temper the milk mixture into the egg mixture by adding two large spoonfuls of batter to the milk mixture and whisking, then pour the milk mixture into the egg mixture while beating on low for 10-15 seconds.

Make Blueberry Batter: Remove ⅓ of the cake batter to a medium bowl. Mix in ⅓ cup cooled blueberry jam and food coloring if desired.

Bake: Layer batter into pans starting with vanilla, then blueberry, then vanilla. Swirl with a knife a few times. Bake for 25-30 minutes until a toothpick inserted in the center comes out clean. Cool in pans for 15 minutes before inverting onto a cooling rack.

Make Frosting: In a saucepan, toast flour and sugar over medium heat for 2 minutes. Add blueberry powder and whisk. Slowly add milk, whisking continuously. Bring to a simmer and cook for 1 minute until thick and pudding-like. Transfer to a heat-proof bowl. Cover with plastic wrap touching the surface to prevent skin formation. Cool completely in the refrigerator or freezer. Beat softened butter on high until fluffy (3 minutes). Whip cream cheese until smooth, then add to butter. Gradually add the cooled flour mixture while beating. Add vanilla and salt.

Assemble: Place a small spoonful of frosting on a cake board. Place a cake layer, cover with frosting, and add the next layer. Frost the top and sides. Apply a crumb coat and chill for 10 minutes (freezer) or 30 minutes (fridge). Apply final frosting coat. Decorate with fresh blueberries and cover the top with remaining jam if desired.