Grease a 9x5 inch loaf pan with some butter and preheat the oven to 350°F.
Wash the lemons thoroughly, then dry and zest them. Juice only half a lemon. Then set both the juice and the zest.
Using a hand mixer, cream together the butter and sugar until light and fluffy. Then, beat the eggs in one at a time.
While mixing, add the sour cream, milk, lemon juice and lemon zest into the cream.
In a separate bow,, combine the flour with the cornstarch, salt, vanilla extract and baking powder. Stir the dry ingredients into the batter.
Pour the batter into the greased loaf pan and bake the cake for 45 minutes in the oven.
In the meantime, make the frosting. Juice another half a lemon and zest it.
Then, mix the sour cream with the powdered sugar and the lemon juice until smooth. Frost the cake and sprinkle with lemon zest and blueberries before serving.
