The horchata sherbet can be prepared up to a month in advance and makes more than you need for these shakes. Try it solo with fresh berries. Recipe adapted from Aran Goyoaga and her blog, Cannelle et Vanille.
In a small saucepan, bring 2 cups rice milk, sugar, almond flour, cinnamon sticks, and pinch of salt to a rapid simmer over medium-high. Reduce heat and gently simmer 10 minutes. Pour rice-milk mixture through a fine-mesh sieve into a small bowl; discard solids. Refrigerate 2 hours (or up to overnight).
In a large bowl, using an electric mixer, beat coconut milk on high until frothy, 5 minutes. Gently stir together whipped coconut milk and chilled rice-milk mixture. Freeze in an ice cream maker according to manufacturer's instructions. Transfer sherbet to an airtight container and freeze 2 hours (or up to 1 month).
In a blender, blend 2 1/2 cups sherbet and 1/2 cup rice milk until frothy. Pour into six chilled glasses and top with pistachios. Serve immediately.