Mix the dry ingredients, knead for 5 mins, then knead in your butter and then the poppy seeds. Knead for a total 8 mins. Cover and leave to rise for about 1 hour, until doubled.
Heat the milk for the custard. Whisk the egg yolk, sugar, cornflour and lemon zesttogether, then slowly whisk in the warm milk.Return to the pan, cook until thick, then stir in the vanilla and butter. Leave to cool.
Divide the dough into 4 balls and leave to prove for 30–45 minutes. Press a deep well into each bun, fill with lemon custard and add a few frozen blueberries if using.
Bake at 220°C (200°C fan) for 10–12 minutes until golden. Brush with melted butter, coat in caster sugar and enjoy warm with a cup of tea.
