Rhubarb Muffins with Crumb Topping are a cinnamon flavored muffin that is grain and sugar free. Tips, Tricks and Substitutions for this Keto Rhubarb Custard Bar Bottom cake layer is moist and lemony, the middle layer is a sweet, yet tart rhubarb custard and the top a Lemon flavored cake with tiny bits of rhubarb scattered throughout, drizzled with a lemon glaze. Keto Rhubarb Berry Crumble is one of those heart warming desserts that make me think of home. Keto Cranberry (Blackberry or Rhubarb) Upside-Down Cake Recipe Topping Ingredients: 2 tablespoons butter, softened 2 tablespoons erithrytol, powdered in a spice/coffee grinder 10 drops