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.
In a small pan combine the sugar and water. Bring to a boil over medium heat, stir to dissolve sugar.
Continue boiling without stirring. Use a pastry brush and some water to wash down the sides of the pan to remove sugar on the sides of pan. Heat until syrup reaches the soft ball stage or 238 degrees. Immediately remove from heat.
In a large mixer beat the eggs briefly on medium speed.
Slowly add the hot syrup in a thin stream, pouring it down the sides of the bowl. Do not allow it to hit the beaters or may splash.
After all the syrup has been added raise speed to medium high speed and beat until the mixture is fluffy and cooled to body temperature. About 15-20 minutes.
Reduce speed to medium low and gradually add the softened butter 2 - 3 Tbsp at a time, beat well between additions.
Add in lemon juice and zest.
To assemble cake place one layer on cake stand and spread with half the blueberry jam. Place second layer on top and spread with the remaining blueberry jam. Add third layer and frost the cake on top and the sides with buttercream. Garnish with blueberries and lemon slices