Edd Kimber's bakery-style blueberry muffins recipe

Ingredients

• 35 g unsalted butter, diced
• 50 g caster (superfine or granulated) sugar
• 1 sprinkle of lemon zest (optional)
• 80 g sour cream, or buttermilk, or yoghurt
• 1 large egg white
• 85 g plain (all-purpose) flour
• 1 tsp baking powder
• 0.2 tsp fine sea salt
• 170 g blueberries