Butter a 1 lb. loaf pan and line it with parchment paper to form a hammock to lift the bread out of the pan later on.
Once the dough has doubled in size (or after taking it out of the fridge after rising overnight), punch it down to remove excess air. Transfer the dough to a clean and lightly floured surface, then roll it out to a 12″ x 16″ (35 x 40 cm) rectangle. If the dough keeps springing back a lot while rolling it out, let it rest for 5 minutes before continuing.
Spread the melted butter over the dough, then sprinkle the cinnamon sugar over the butter.
Using a knife or pizza cutter, cut the dough in half once through the longer side, then slice each piece into 8 pieces. Roll up the mini cinnamon rolls.
Layer the cinnamon rolls into the loaf pan. I started out with placing some rolls standing up facing the sides of the pan, then placed some face up in the middle of the pan. After that, randomly but evenly distribute the rest of the rolls in the pan.
Lightly cover the pan with a towel, and let the loaf rise in a warm environment until doubled in size (~30 minutes).
Preheat the oven to 350°F / 175°C. Make an egg wash by whisking together an egg with 1 tbsp. milk, then brush it over the rolls (optional, but recommended for browning.)
I recomend placing the loaf pan on a flat baking sheet to catch any potential sugar spills in the oven. Bake the loaf for 35-40 minutes. Use a thermometer to check when the rolls are done cooking (200°F / 93°C.)
Allow the cinnamon roll loaf to sit in the hot loaf pan for at least 10-20 minutes before transferring it out.