Gently melt the butter and milk together in a pan over a low heat until the butter has melted. Set aside until lukewarm (about 40°C/100°F).
Mix the flour, yeast, caster sugar, salt and cardamom together in a large bowl. Make a well in the centre and pour in the beaten egg and yolk followed by the warm milk and butter mixture. Use a wooden spoon to mix until you have a slightly sticky dough. When the dough has taken shape, turn out onto a floured surface and knead for about 10 minutes until smooth and springy. (Alternatively knead in a stand mixer with a dough hook for 4-5 minutes.) Dust with flour if you find the dough too sticky.
Transfer the dough to a clean greased bowl, cover with a damp tea towel and let it rise in a warm place for 1 hour, or until it has doubled in size.
For the filling, beat the unsalted butter, soft light brown sugar and cinnamon in a bowl until you have a smooth paste.
When the dough has risen, punch it down in the bowl and tip onto a lightly greased work surface. Roll into a rectangle, about 45 x 30cm (17 x 12in) and about 5mm (¼in) thick. Spread the filling all over.
Starting from one long side, roll the dough into a cylinder. Use a sharp knife to slice into 12 equal-sized pieces. Place the rolls in a lightly greased roasting tray (about 25 x 20cm/10 x 8in), cut-side up. Cover with lightly greased cling film and leave to rise for around 30 minutes in a warm place until puffy.
Preheat the oven to 200°C/180° fan/400°F/Gas 6. Brush the buns with the egg, sprinkle with the sugar and bake for 15 minutes, or until golden brown.
Leave the buns to cool in the tin for 10 minutes, then transfer to a wire rack to cool a little before serving.
Notes
Sweet bread dough rolled up with a swirl of sweet buttery cinnamon throughout. What's not to love?