Whisk together warm water, yeast and 1/2 teaspoon of sugar to activate the yeast; wait 5 minutes until it bubbles.
Add 1 egg, vanilla and butter to the liquid yeast mixture.
In the bowl of a food processor or stand mixer with the hook attachment, add the salt, sugar, freshly grated nutmeg and 3 cups of flour. Slowly add the liquids to the flour mixture and begin processing until a ball is formed. If needed, add the last cup of flour, ¼ cup at a time.
Dust counter with flour and finish kneading on the counter with a little flour. The dough will be a little sticky.
First Rise: Put dough in a buttered large bowl and let rise covered with a dish towel or plastic wrap, doubled in size, about for 1-1.5 hours (exact time is weather dependent). If the dough feels tough, let rise longer. Punch dough down and let it rest for 15 minutes.
Shaping: Transfer dough to a lightly floured surface and roll into a 13(ish)-inch square. No perfection necessary. Spread the cinnamon butter mixture over the dough with a spatula and tightly roll dough up to form a 12-inch log. Seal the edges as best you can and place seam side down.
Cut into 1-inch slices with a serrated knife or floss. You’ll should get 12 pieces. Snugly arrange cinnamon rolls in a greased baking pan or round cake pan (use 2 if they’re small).
Second Rise: Cover rolls with foil and refrigerate overnight, or up to 48 hours.
To bake and serve, preheat oven to 350 degrees. Take cinnamon rolls out of the fridge and let come to room temperature while oven heats up. Brush the tops with egg wash (1 egg + 1 tbsp of water) and bake for 25-30 minutes, until golden brown on the edges. Allow to cool for 10 minutes, spread frosting over the top and serve while still hot.