Preheat oven to 350 degrees.
Grease and flour 3 eight inch cake pans and line bottom with wax paper.
Cream together butter, sugar, lemon zest and lemon extract until fluffy.
Add in the egg whites gradually, a couple at a time. Beat well between additions and scrape down sides of bowl.
Mix together in another bowl the flour, baking powder and salt.
Alternating flour and milk, beat the dry ingredients and milk into the butter mixture. Beat on medium speed for one minute to eliminate any lumps.
Set aside 1 cup of batter in a small bowl.
Divide the remaining batter between the three cake pans, smooth top with spatula.
Add 2 1/2 Tbsp jam to the reserved batter and mix well.
Drizzle by the teaspoon over the batter in the pans.
Use a wooden skewer to swirl the blueberry mixture into the lemon batter without mixing it in.
Bake for 25 minutes or until toothpick comes out clean and cake pulls away from the sides.s
Let cool for 10-15 minutes and remove from pan onto a wire rack. Cool completely.