Stir the yeast into the lukewarm milk and water with a pinch of sugar and leave for 10 minutes until frothy.
Mix the flour, salt and sugar in a large bowl, pour in the yeast liquid and knead for 5-6 minutes into a smooth, elastic dough (don't overwork it, or it will be hard to roll).
Cover and leave to rise at room temperature for 1 hour, then flatten it, wrap in cling film and chill for at least 1 hour (or overnight).
Soak the raisins in the rum or hot tea so they plump up.
Make the custard: bring the milk to the boil with the vanilla. Mix the yolks with the sugar and cornflour until smooth, pour in half the hot milk in a thin stream, stirring constantly, then pour it all back into the pan and cook over medium heat, stirring, for 1-2 minutes until it becomes a thick, glossy pudding. Cover with cling film and leave to cool.
Put the cold butter between two sheets of baking paper, beat it with a rolling pin and flatten it into a square of about 18×18 cm, then return it to the fridge — the butter and dough should be similarly cold and pliable.
Roll the dough out on a floured surface into a square of about 26×26 cm, put the butter slab in the middle turned 45°, fold the corners of the dough over it like an envelope and pinch them together.
Roll it into a long rectangle of about 20×60 cm, then fold it in three like a letter (the first 'tour'). Wrap in cling film and chill for 30-45 minutes.
Repeat the rolling and folding in three twice more, turning the dough 90° each time and chilling for 30-45 minutes after each turn. If the butter threatens to break through the dough, chill it again straight away.
After the last chill, roll the dough into a rectangle of about 30×40 cm, 4 mm thick, and spread the cold custard thinly and evenly over it, leaving 2 cm free along one long edge.
Scatter over the drained raisins, roll up tightly from the long side and chill the roll for 20 minutes so it's easier to slice.
Cut into 3 cm thick swirls with a sharp knife, lay them on a tray lined with baking paper (tucking the ends underneath) and leave to rise covered for 1.5 hours.
Preheat the oven to 190 °C and brush the tops of the swirls with beaten egg.
Bake for 18-22 minutes until golden brown. If you like, brush the still-hot swirls with warmed, thinned apricot jam to make them shine.
A Receptnaptár original recipe. Calories and protein are estimated.