Labneh Frosting
The night before: Line a large sieve with cheesecloth and set over a bowl so that it rests at least an inch off the bottom. Place the yogurt in the cheesecloth. Cover and put in the fridge to let drain overnight. The next day take the strained yoghurt from the fridge and discard the whey. Spoon the labneh into a bowl, add the vanilla and whisk together, set aside.

Loaf
Preheat the oven to 350F. Grease and line a 9×4 inch loaf pan with baking paper.

Toss the blueberries in the tbsp of flour.

In a bowl or stand mixer cream the butter and sugar together with an electric whisk until light and fluffy, about 2 mins. Add the eggs one at a time, beating in till fully combined. Add the sour cream, lemon zest, juice and vanilla and mix till fully combined. In a bowl whisk all the dry ingredients together. Add to the wet ingredients and mix till just combined. Add the blueberries and fold into the batter carefully, do not overmix.

Spoon the batter into the loaf pan and bake for 60mins or until a toothpick into the middle comes out clean. Allow to cool for 10mins and then transfer to a cooling rack. Let cool completely.

Spoon the frosting over the top of the loaf, using a palette knife to swirl it all over.