Giant and fluffy overnight sourdough cinnamon buns loaded with brown sugar and walnuts. Everyone will want seconds of this pantry staple breakfast! These brioche cinnamon rolls are warm and delicious, a perfect brunch treat for hungry families or guests.
In a large mixing bowl, add the sourdough starter, flour (start with 4 1/2 cups), sugar, active yeast, and salt. Mix well to combine, and make a well in the center of the flour mixture.
Add melted butter, milk, and egg to a bowl, scramble well with a fork until combined. Add the egg mixture to the center of the flour well. Pour the warm water over the egg mixture. With a large spoon, mix well to combine all ingredients into a dough. Dough should be sticky and thick. If your dough is sticking to the sides of the bowl, add the remaining 1/2 cup flour.
Cover with plastic wrap and place bowl in the refrigerator overnight (8 to 12 hours).
The next morning, grease 2 9x9 inch loaf pans. Turn dough out onto a well-floured work surface.
Roll out the dough into two large rectangles.
In a saucepan, heat the butter, brown sugar, and cinnamon and stir until well combined. Pour the cinnamon mixture over the doughs, and use a spoon or pastry brush to make an even layer over the top of the dough. Sprinkle on the walnuts and ground flaxseeds.
Take the long side of the dough, and tightly roll the edges towards the other side, roll all the way until you have a long log of dough. With a sharp knife, cut the buns into 2 inch slices. Place the slices in your greased loaf pans (mine fit 9 in each).
Preheat oven to 375 degrees Fahrenheit. Bake rolls for 30-45 minutes (mine took the full 45). Tops should be brown, but if it starts to look burnt, then cover with foil and continue to bake.
Allow buns to cool on a rack for at least 20 minutes before enjoying. Top with icing, butter, or powdered sugar if desired.