Preheat the oven to 375F. Grease and flour two 9" cake pans. I also like to cut a round parchment circle and place it in the bottom of the pans.
Beat the butter and sugar in a large bowl and set aside. In a separate bowl, mix together the flour, baking powder and salt.
Crack the eggs and add the vanilla into the butter-sugar mixture and beat well. Add in half of the flour mixture, and mix until incorporated.
Add the milk to the butter-sugar mixture, and then follow with the remaining flour mixture. Beat until smooth and no lumps remain in the batter.
Divide the batter between the two cake pans and bake for 30-38 minutes until an inserted toothpick comes out clean.
Cool on a baking rack completely before frosting.
How to make the vanilla buttercream frosting:
Beat the butter and sugar together until smooth, then add in the vanilla and 2 Tablespoons of cream. Adjust the consistency with more sugar for a stiffer frosting, or more cream for a softer frosting.
How to make the blueberry pie topping:
Place all ingredients except for the lemon juice and salt in a pot set above medium heat.
Stirring frequently, cook for 8 minutes until the blueberry skins have dissolved and the consistency resembles a loose jam.
Remove from heat and add the lemon juice and pinch of salt. Allow to cool before adding to the cake.
To assemble the complete Blueberry Pie Cake:
Level the cakes using the toothpick method or using a cake leveller.
Place one cake half on the serving platter. Apply frosting to the top of the cake and spread to the edges.
Top with a 1/2 cup of the blueberry filling and use an offset spatula to push the blueberry mixture to about 2" away from the edge of the cake, to prevent the filling from mixing with the outer frosting when you add the top cake layer.
Carefully center the top cake layer on the bottom, and then apply a thick coat of frosting to the whole cake. This will serve as your crumb coat.
Allow this frosting layer to air out and harden slightly for a couple hours. Then, apply a second coat of frosting, to be the outer layer. You're going to want to ensure this layer is smooth and consistent.
Pour the remaining blueberry pie topping overtop of the cake. Allow it to spread and drip down the sides - it just looks more delicious that way!
Cut and serve. If not consuming within the next 24 hours, store the cake in the refrigerator.