Preheat oven to 350F. Grease and flour two 8″ x 8″ square cake pans.
In a large bowl, sift the flour, baking powder, baking soda and salt.
In another bowl, whisk sour cream, lemon zest, juice and vanilla.
Beat butter and sugar together on medium speed in a bowl of a standing mixer until light and fluffy. Beat in eggs, one at a time, until blended.
Reduce the speed to low and beat in 1/3 of the flour mixture, followed by half of the sour cream mixture. Repeat with another 1/3 of the flour mixture and the remaining sour cream mixture. Beat in the remaining flour mixture until just incorporated.
Divide batter evenly among prepared cake pans. Transfer pans to oven and bake for 20 to 25 minutes or until wooden pick inserted in cake comes out clean. Cool in pan on wire rack for about 10 minutes before inverting on rack. Let cool completely.
To prepare blueberry sauce, mix all ingredients in a small bowl and bring to a boil. Reduce heat and simmer until mixture begins to thicken, about 2 minutes. Remove from heat. Let cool completely.
Beat heavy cream with powdered sugar until the mixture holds stiff peaks. Place into the fridge to chill.
To assemble, slice cakes into half horizontally, so you have 4 layers of cake. Then cut vertically to have 8 layers of rectangular cakes, if you want to make two cakes.
Place one cake layer on the serving late and spread with blueberry sauce. Gently spread the whipped cream on top, and scatter with blueberries. Top with another cake layer and repeat the same process until you place the 4th cake layer.
Frost the top and sides of the cake with the remaining whipped cream. Press white chocolate around the sides of cake and place blueberries on top. Refrigerate until ready to serve.
Before serving, sprinkle powdered sugar on top.