Spray the inside of a 6-quart crockpot with cooking spray.: You will notice a faint, neutral scent from the spray as it glazes the pot, creating a barrier to prevent sticking. This simple coating ensures that the edges of the cinnamon rolls can brown a touch without tearing when you scoop them out. If you skip this, pieces can cling to the sides and break apart when serving, so be sure to fully coat the base and sides.
Place cinnamon roll quarters evenly in the bottom of the crockpot.: The soft, pillowy quarters should sit snug but not packed, and you may hear a slight squish as they nestle together. Arrange them so there are no large gaps, which helps the custard soak uniformly. If pieces overlap too tightly, the center may remain doughy, so give them a moment to settle in an even layer.
In a medium mixing bowl, add the eggs, heavy cream, brown sugar, vanilla extract, cinnamon, and nutmeg. Whisk together until well combined.: As you whisk, the mixture will develop a silky sheen and a sweet, spiced aroma will rise. The blending of the eggs and heavy cream creates the custard base that hydrates the dough; ensuring it is smooth prevents scrambled bits. A common mistake is under whisking, which can leave streaks of yolk or uneven spice pockets, so whisk until uniform and slightly aerated.
Pour the egg mixture evenly over the top of the cinnamon rolls.: You should hear a gentle patter as the liquid flows over the dough, and visually the rolls will begin to bloom as they absorb the custard. Pouring evenly ensures each bite gets custard, which yields a tender, cohesive texture after cooking. Avoid pouring only to one side, as that can make some pockets overly soggy and others too dry.
Place the lid on the crockpot and cook on low for 3 hours. Cinnamon rolls should be golden brown along the edges and no longer gooey in the middle.: During the long slow cook you will notice a comforting bakery like aroma filling the kitchen, and the edges will take on a warm golden tone. The internal texture should be set yet soft when a toothpick comes out cleanish, and the center should not be gelatinous. A frequent error is lifting the lid too often to check, which lets heat escape and extends the cook time, so resist peeking unless necessary.
Once done, spoon the icing over the top of the cinnamon rolls.: The frosting will melt slightly on contact, creating glossy ribbons and releasing a tangy sweet scent that complements the spices. Use a spoon to allow the icing to pool into crevices, boosting visual appeal and textural contrast. If you apply it while the casserole is too hot, the frosting will thin excessively, so wait a few minutes off heat for the best balance.