Melt the butter in a saucepan, pour in the milk and let it warm until lukewarm.
Meanwhile, whisk the eggs and sugar until pale, thick and fluffy.
Add all the dry ingredients, the milk and butter mixture, lemon juice and lemon zest, and whisk everything together quickly but thoroughly - in a couple of batches if needed.
Spoon the batter into muffin cases, and scatter the blueberries and white chocolate over the top.
Using the handle of a spoon or similar, swirl the filling through the batter so it is mixed in.
Bake the muffins in a preheated conventional oven (top and bottom heat) at 190°C for about 20-22 minutes.
Let them cool before serving, and dust with icing sugar if you like.