Cinnamon Roll Filling:
While the dough is rising, prepare the cinnamon roll filling. In a medium-sized bowl, mix together the light brown sugar, ground cinnamon, and cornstarch. Set aside.
Funfetti Cinnamon Roll Dough:
Preheat your oven to 200°F (95°C).
In a large bowl or the bowl of a stand mixer, combine the flour, instant dry yeast, and salt.
In a separate, heat-proof bowl, combine the milk, softened butter, and granulated sugar. Heat in the microwave for 60-90 seconds until warm and the butter is melted. Stir to combine.
Slowly add the warm milk mixture into the flour mixture, mixing on medium speed with a dough hook.
Add the beaten 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 elastic. If the dough is too sticky, add additional flour 1 tablespoon at a time until the dough begins to leave the sides of the bowl.
Once the dough reaches a tacky but not sticky consistency, cover the bowl with plastic wrap and let the dough rest for 10 minutes.
Generously flour your counter and roll out the dough into a large rectangle, about 12 inches x 18 inches and 1/2 cm thick.
Sprinkle 1/2 cup of rainbow sprinkles over the dough and gently press them in. Fold the dough in thirds (like a letter) and continue rolling it out.
Heat the remaining butter in the microwave for 10-15 seconds to partially melt it. Pour it over the dough and spread it evenly, leaving about 1/2 inch of dough on one long side uncovered.
Sprinkle the cinnamon roll filling evenly over the buttered area and pat it down gently with a spoon or spatula.
Tightly roll the dough toward the uncovered strip, sealing the edge. Cut the dough log into 9 equal pieces using dental floss or a sharp knife.
Rise the Rolls:
Place the rolls in a greased 8×8 inch square pan. Cover with foil and place in the preheated oven for 30 minutes to rise. TURN OFF the oven during this time.
After 30 minutes, remove the pan from the oven and uncover it. Preheat the oven to 350°F (175°C). Once preheated, bake the rolls for 26-32 minutes until golden brown.
Vanilla Frosting:
In a medium-sized bowl, combine the room temperature butter, vanilla extract, almond extract, and salt. Mix with a whisk or hand mixer until smooth.
Gradually add in the powdered sugar, then pour in the milk. Mix until smooth and creamy.
Finish and Serve:
Allow the cinnamon rolls to cool for 10-20 minutes. Spoon the vanilla frosting over each roll and serve warm.