Add 3 egg yolks to a medium bowl and whisk together. Set aside.
Measure out 3 cups oat milk, and pour 1½ cups to a medium pot. Reserve the second half for later.
To the pot with 1½ cups oat milk, add ¼ cup cashew butter, ⅓ cup sugar, ¼ tsp salt, ⅛ tsp cinnamon.
Heat the saucepan over medium-low heat, stirring until the cashew butter melts and the mixture is smooth. Cook while stirring until mixture simmers at the edges, but do not allow it to come to a full boil.
Remove from the heat. Take ⅛ cup of the heated milk at a time, and add it to the yolks whisking constantly. You want to bring the temperature of the eggs up slowly to avoid scrambling them. Add it in ⅛ cup at a time until all the mixture has been whisked in.
Return the yolk and milk mixture to the saucepan and heat again over medium low heat for approximately 3 minutes. Stir constantly, until the mixture thickens enough to coat the back of a spoon. Do not allow the mixture to boil.
Strain the thickened mixture through a fine mesh strainer into the bowl with the remaining 1½ cups oat milk. Stir in ⅓ cup maple syrup and 2 tsp vanilla.
Allow the mixture to come to room temperature, then refrigerate at least 2 hours up to overnight.
Ice cream maker: Freeze in your ice cream maker, according to instructions. For my kitchenaid, I make sure to freeze the bowl at least 8-12 hours before use. Then pour in the cold ice cream mixture and mix for 15-20 minutes. It should start to thicken. Add hokey pokey or mix-ins during the last few minutes. Pour into a loaf pan and freeze for 3-4 hours.
By hand: Pour the chilled mixture into a loaf pan and freeze for 30 minutes. Once it begins to freeze (check the crystals near the edges of the pan), stir vigorously with a spatula. Return to the freezer. Repeat every 30 minutes for 2 hours or until ice cream is frozen through.