As prep, grease an 8 x 8 baking dish (or 9-inch round). Combine all of the custard ingredients until smooth and set aside. To make the cinnamon roll filling, soften the butter substitute in the microwave and combine it with the 1.5 tsp of cinnamon and ¾ cup of brown sugar replacement.
Use a rolling pin to roll out the bread slices, so they’re thinner and easier to roll up. Line up rows with three bread slices slightly overlapping each other (refer to image). Work in batches or make all of the rows at once. You’ll be making five rows total and reserving one extra slice for later.
Spread the cinnamon filling over the flattened bread slices. Start with one row of 3 slices and carefully roll it up as if it is a cinnamon roll. Cut it in half widthwise, dunk it in the custard mixture, and place each half facing up in the baking dish. Continue the process until all of the rolls are soaked and assembled in the pan. Pour the extra custard mixture over the rolls, focusing on the corners and sides (so they don’t dry out).
Allow the casserole to sit in the fridge for 15 minutes minimum while the oven preheats. Preheat the oven to 350F. After the casserole rests, press down the cinnamon buns so there are fewer gaps. Break the extra bread slice into pieces and fill the gaps, soaking them with the custard.
Bake the casserole, covered with foil, for 20 minutes. Remove the foil and continue to bake for 25-30 minutes, until the custard is set and the top is golden brown.
Make the frosting by using the recipe as a base. It will be thick. Add more yogurt or more milk depending on if you want it thinner, like icing, or thicker, like frosting and serve it with the casserole.