Heat oven to 200C/180C fan/gas 6 and line a 12-hole muffin tin with paper cases. Beat the butter and caster sugar together until pale and fluffy. Add the eggs and beat in for 1 min, then mix in the yogurt, vanilla extract, milk and mashed bananas. Combine the flour, baking powder and bicarb in a bowl with 1/4 tsp fine salt, then tip this into the wet ingredients and stir in. Finally, fold in the blueberries and divide the mixture between the muffin cases.