To make the dough, add milk and butter to a saucepan or microwave-safe dish, then gently heat the milk and butter over low heat until the butter is molten. If using the microwave, heat the milk and butter for 1.5 minutes, but make sure to keep an eye on it as milk and butter boil very quickly.
In a mixing bowl, combine flour, yeast, sugar. Add the warm milk and butter mixture. Mix well using a spoon, until a smooth dough forms. (Note: If using an electric mixer, use the kneading attachment on low, then gradually increase speed to highest setting within 5 minutes.)
Cover bowl with clingwrap and let dough rise in a warm spot for 60 minutes until it has doubled in size.
Preheat oven to 180°C (fan-forced oven) and grease a muffin pan.
For the filling, combine the softened butter, sugars and cinnamon in a bowl. Mix well until you have a thick cinnamon paste
On a lightly floured surface, gently knead the dough again and then roll-out the dough, into a 0.5cm thick rectangle (approx. 50x40cm).
Evenly spread the cinnamon paste over the dough, leaving 3cm on one of the short edges bare.
Fold the dough, starting with the short side covered with cinnamon spread. Keep rolling it towards the bare edge.
Using a knife, or some dental floss (see notes below), cut the dough into 12 evenly sliced swirls.
Place swirls into muffin tin.
In a small bowl, mix the egg yolk and milk until well combined. Then use a pastry brush to brush yolk-milk mixture over each swirl.
Sprinkle pearl sugar (if using) over the cinnamon swirls, then bake the swirls in preheated oven for 20 minutes until golden.
Remove from oven and let cool on a rack for at least 10 minutes before serving. Can be enjoyed warm or cold.