To blanch almonds: bring a saucepan of water to a rolling boil. Add almonds and boil for 45 seconds, then drain and rinse with cool water. The almonds should easily slip out of their skins when squeezed. You can also substitute whole or slivered almonds which have already been blanched and skip this step.In a spice grinder or a clean coffee grinder, pulverize rice and cinnamon stick until finely ground. Add ground rice to a large mason jar or bowl. Add almonds and cover with 2 cups water (use filtered water if your tap water is iffy). Cover and refrigerate 10-12 hours or overnight.The next day, pour the contents of the jar into the canister of a blender, along with 1 more cups of filtered water. Blend on the highest speed for 1 to 2 minutes or until completely smooth.Pour mixture through a fine mesh sieve lined with two layers of cheesecloth, or, if you have one, a nut milk bag is ideal for this. Let most of the liquid drain through, then gather up the edges and squeeze the remaining pulp to force out as much liquid as you can. Discard leftover pulp. You should have about 2 1/2 cups of liquid.Pour horchata into a saucepan along with sweetened condensed milk. Warm over medium heat until it just starts to bubble (do not let it come to a full boil).In a small bowl, whisk egg yolks together with cornstarch and salt until smooth and slightly lightened in color. Slowly ladle in 1/4 cup of warm horchata, whisking to incorporate. Continue to add horchata, 1/4 cup at a time, whisking constantly, until about half of the horchata has been incorporated and mixture is warm to the touch. Pour back into saucepan with remaining liquid. Continue to cook over medium heat until mixture thickens slightly and coats the back of the spoon, about 5 to 7 minutes (it should read 170 degrees on an instant-read thermometer). Remove from heat and transfer to a heat-proof bowl set inside an ice bath. Stir until cooled to room temperature, then cover and refrigerate for at least 6 hours or overnight until completely chilled.Churn in an ice cream machine according to manufacturer’s instructions. Transfer to a freezer-safe container and freeze until firm, or enjoy immediately as soft-serve consistency.