Combine almond flour, coconut flour, sweetener, baking powder, and xanthan gum in medium bowl.
Whisk together butter, whipping cream, juice from the lemons, zest from lemons, and egg in large bowl.
Add dry mix to wet mix and combine until well blended. Spread into 2-4 quart slow cooker crock lined with aluminum foil or a liner. It is noted in the comments that acidic food shouldn't come in contact with aluminum foil so the liner is likely the best choice.
Combine all topping ingredients in bowl and pour over cake batter in crock.
Cover crock pot and cook on high setting about 2-3 hours or until toothpick inserted in center comes out clean.
Serve warm with fresh fruit and whipped cream, if desired.
Use fresh lemons. I know it may be tempting to buy the bottled juice, but this isn't one of those recipes where you can get away with that. Lemon—and lemon zest—is the star of the show here.Line your slow cooker with aluminum foil. As you'll see below, making a crockpot cake is a total cinch, but the foil will help keep the cake moist.A smaller crockpot works best for this cake. If you want to make this in a larger slow cooker, I suggest doubling the recipe.You can also make this keto lemon cake in the oven. Use an 8-inch round and bake it at 325-350 degrees until a toothpick inserted comes out clean.Experiment with toppings. I like this keto lemon cake served warm with whipped cream and sliced strawberries or blueberries. But I've also enjoyed this cake cold topped with sugar-free chocolate syrup!