Prepare a double boiler. Fill a medium pot halfway with water and place over high heat. Set a metal bowl on top but don't let it touch the water.
While water comes to a boil, make the custard for the ice cream: In a separate bowl, whisk horchata, whipping cream, vanilla, 1/2 teaspoon cinnamon, and 1/2 cup sugar until well combined. Set aside.
In the metal bowl of the double boiler, whisk egg yolks for about 5 minutes or until lightened in color and starting to thicken. Adjust water temperature, if necessary, so yolks do not scramble. Remove bowl from heat.
Whisking constantly, slowly add horchata mixture to eggs. Whisk 1 minute after mixtures are completely incorporated so sugar is dissolved. Strain through a fine‑mesh sieve into a bowl. Cover and chill.
While the mixture cools, use a large cookie cutter to cut 3- to 4-inch rounds from tortillas.
Spray tortillas with non‑stick cooking spray and coat both sides with a mixture of remaining cinnamon and sugar.
Push tortilla rounds down into muffin tins to form a bowl or cup and bake at 350 degrees Fahrenheit for 15 to 20 minutes, or until browned and crispy.
While tortilla shells bake, prepare ice cream according to the manufacture's directions. Depending on the machine, ice cream should churn for about 15 to 20 minutes and have a soft-serve consistency. Place in the freezer once extracted from the machine for ice cream to firm up.
Remove shells from the oven and cool completely before filling with horchata ice cream.
Serve with a sprinkle of cinnamon sugar or caramel if desired.