1Whisk together flour, sugar, baking soda and baking powder in a large mixing bowl.
2Add butter, egg whites, vanilla, sour cream, milk, lemon juice and zest and mix on medium speed for a few minutes until smooth then fold the blueberries into the batter, Spoon silicone cupcake holders a little more than half way.
3Bake for 1 hour 20 mins on high with tea towel under lid. ( My sc cooks fast... haha, if that makes sense so keep an eye on them )
5Best the unsalted butter for around 40 seconds then add :
6two cups of icing and mix until smooth.
7Add lemon juice, lemon zest vanilla and 2 tbs of cream and mix until smooth.
8Add remaining icing and cream then mix until smooth.