Bring almond milk and cream to a simmer in a medium saucepan. Meanwhile, whisk sugar, cornstarch, flour, and salt in a large mixing bowl. Add the egg yolk to the sugar mixture and whisk until smooth but very thick. While whisking constantly, gradually add the warm almond milk mixture to yolk mixture. Once the 2 mixtures are fully combined, return everything back to the saucepan and cook over medium-low heat, whisking constantly, until thick, about 2–3 minutes. Add the butter and vanilla and stir until the butter has melted.