Mix egg, sugar, salt, lemon juice, lemon zest and vanilla in a mixing bowl and whip on high for a few minutes. It should triple in size.
Fold through the flour and once combined fold in the melted butter. Mix until it is a smooth batter.Quickly roll the cranberries through some icing sugar and then add them to the cake batter. Pour everything into a cake tin lined with baking paper.
Bake for about 45-60 minutes. Make sure to check if the cake is cooked through by inserting a wooden skewer in the middle. If it comes out clean it is done.