Preheat your oven to 200 F / 95 C.
In a large bowl or the bowl of a stand mixer, combine 3 1/2 cups of flour, 1 packet of instant dry yeast, and 1 tsp salt.
In a separate, heat-proof bowl, add 1 cup milk, 1/4 cup softened butter, and 1/2 cup granulated sugar into a microwave-safe bowl.
Heat on high power for 60-90 seconds, until the mixture is warm to the touch and the butter is melted. Stir to combine the ingredients.
Add the warm milk mixture slowly to the flour mixture, mixing on medium speed with a dough hook between additions.
Add 1 large egg and mix on low speed until a sticky dough ball forms. Continue mixing on medium-low speed for 2-3 minutes until the dough becomes more elastic.
As the dough mixes, add additional flour 1 Tbsp at a time until the dough begins to leave the sides of the bowl. My favorite way to know if my dough is ready is to poke it with my finger. It should feel tacky but not sticky. As you pull your finger away, there should be no residue on your finger.
Once the dough reaches this stage, cover the bowl with plastic wrap and let it rest for 10 minutes. This will relax the gluten and make the dough easier to roll out.
Generously flour your counter and roll out the dough ball.
When you're about halfway through rolling, sprinkle the dough with 1/2 cup of rainbow sprinkles and gently press them into the dough.
Fold the dough in thirds (like a letter) and continue to roll it into a large rectangle 12 inches x 18 inches. It should be about 1/2 cm thick.
Sprinkle the remaining 1/2 cup of rainbow sprinkles on top of the dough and gently roll or press them into the dough.
Heat the remaining 1/4 cup of room temperature butter in the microwave for 10-15 seconds, to partially melt it.
Pour the partially melted butter over the dough, and gently spread it over the sprinkles with a pastry brush or the back of a rubber spatula. Be sure to leave about 1/2 inch of dough on the long side of the rectangle uncovered. This will help you seal off the roll and keep it tight.
Spread the cinnamon roll filling (instructions above) evenly over the buttered area and pat down with the back of a spoon or spatula.
Tightly roll the dough horizontally, toward the uncovered strip.
Cut the dough log into 9 equal pieces with dental floss (you can cut it into more pieces if you want smaller/shorter cinnamon rolls), and place in a greased
square pan (mine is 8in x 8in).
Turn the oven off! I REPEAT, TURN THE OVEN OFF.
Cover the cinnamon roll pan with foil and place in the oven for 30 minutes (until the cinnamon rolls have risen).
Remove the pan from the oven and take off the tinfoil. Turn the oven back on, and heat to 350 degrees.
Once the oven has preheated to 350 F, bake for 26-32 minutes or until rolls are golden brown.