Add the active leaven, egg, milk, and sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed until the leaven is fully dissolved and the mixture looks airy and frothy.

Switch from the paddle attachment to the dough hook and add the flour to the mixing bowl. Mix on low speed for a couple of minutes until the flour is fully incorporated. Cover the bowl with a damp tea towel and let it rest on the counter for about 30 minutes. This allows the flour to fully absorb the liquids and jumpstarts the gluten development. While the dough is resting, take the butter out of the fridge and cut it into small cubes.

Add the salt and knead the dough on low-medium speed for a couple of minutes. Then, while the mixer is running, add the softened butter cubes, one by one, until the butter is fully incorporated. Increase the speed to medium-high and knead the dough until it becomes smooth and elastic. It may take about 10 - 15 minutes to develop sufficient dough strength. The dough is fully developed when it easily releases from the sides of the mixing bowl and clings to the dough hook. After kneading, the dough should feel soft and smooth. It will have a slightly shiny surface and may feel a bit tacky, but it should no longer be sticky.

After mixing, place the dough in a lightly greased bowl and cover it with a damp tea towel or plastic wrap. Let it bulk ferment in a warm spot (ideally between 26 – 28 degrees) for about 4 hours. By the end of the bulk fermentation, the dough should look aerated and contain many small bubbles, but it doesn’t have to double in size during this time. After the bulk fermentation, place the dough in the fridge for a cold overnight rest.

The next day, take the dough out of the fridge and let it come to room temperature. Meanwhile, soften the butter and combine the sugar and cinnamon for the filling. Turn the dough out onto a floured work surface and roll it out into a rectangle (about 45 x 30 cm). Spread the softened butter evenly over the dough and sprinkle over the cinnamon-sugar mixture. Starting from one short side, roll the dough into a tight log.

Using a thin piece of thread or a very sharp serrated knife, cut the log into 10 equally sized cinnamon rolls. If you are using a thread to cut the individual pieces, slide it under the roll. Then, holding both ends of the thread, bring it around the dough roll and cross it over the top. Pull both ends of the thread in opposite directions until it has fully cut through the dough. Then, reposition the thread and repeat the process with the remaining rolls.  Using a thread to cut the rolls is an effective way to achieve a clean, even cut and prevents squishing the rolls too much.

Line a baking dish with parchment paper. Place the cinnamon rolls into the baking dish, and make sure to leave a little space between each roll to allow them to rise and bake evenly. As they proof and expand, they will naturally come into contact with one another, creating the classic pull-apart effect. Cover the baking dish with plastic wrap and let the rolls proof in a warm spot (28 °C - 30°C) for about 2 hours, or until they have risen and almost doubled in size. For a light and fluffy texture, it’s crucial to let the rolls rise until they look puffy and airy.

Pre-heat the oven to 180° C. Brush the top of the rolls with egg wash and bake them for about 30 minutes until lightly golden brown. If the top browns too quickly, you can loosely tent the baking dish with aluminum foil. Take the rolls out of the oven and let them cool in the pan for about 30 minutes. Lightly dust them with powdered sugar before serving.