Gather ingredients and equipment. Fill the larger bowl (big enough to fit the custard bowl inside) half way with ice and add enough water to make an ice bath. Place the smaller bowl and sieve by the stove.
Cook the coconut milk in a small sauce pan over medium heat until simmering.
Meanwhile, mix the cornstarch (or arrowroot powder), salt and xylitol together in the medium bowl. Add the whole egg and egg yolks whisking thoroughly together. When the coconut mixture is hot and steaming, pour it slowly into the egg mixture, whisking all the while. Because the xylitol gets cold as it melts, don't be too concerned about scrambling your eggs at this time. Pour the mixture back into the sauce pan. Place the mesh sieve over the bowl to use shortly.
Whisk the custard mixture continuously over medium-low to medium heat. Because the xylitol is still melting, it takes the mixture several minutes to get hot. DO NOT STEP AWAY FROM THE STOVE. Be patient and whisk the custard the whole time until it becomes very thick like a pudding. Continue cooking for one minute more to cook out the taste of the starch. (Be ready to take the pot off the heat if it looks like the eggs might scramble. This step takes concentration and at times, furious whisking.)
Pour the custard into the sieve and work it through with a rubber spatula to remove any bits of cooked egg. Place the custard bowl into the ice bath and hold it carefully so as not to get any water in it. Stir the custard until COMPLETELY COOL. Stir in 1 teaspoon of vanilla. Set aside.
Measure the palm oil shortening into a medium mixing bowl and mix it with a hand mixer until soft. Add the butter and mix until completely incorporated. Set aside. (If using ghee: Warm the ghee or vegan butter gently. Add the ghee to the shortening and mix completely.) NOTE: The shortening must be soft to incorporate into the custard. * if using all shortening, add butter flavoring extract to taste.
Whipping the frosting
Scrape the cold custard into the bowl of the stand mixer, and using the whip attachment, mix on medium for several minutes to incorporate air into the custard. Slowly add the shortening mixture to the custard 2 tablespoons at a time, waiting until each addition is completely incorporated before adding more. When half of the shortening is incorporated, start alternating adding in the Swerve sweetener. Continue until all of the shortening is incorporated. It takes quite some time.
Begin adjusting the flavor of the buttercream frosting by adding the vanilla 1/2 teaspoon at a time. This is also the time to adjust the sweetness. I use stevia glycerite to add additional sweetness to avoid the pronounced cooling sensation one gets from sugar alcohols.
Use right away, refrigerate up to a week or freeze. It keeps forever in the freezer. Just put it in the refrigerator the night before to thaw and leave it on the counter 30 minutes before use. Whip it before using.
Makes 5 cups of frosting. NUTRITION PROVIDED IS PER 1/4 CUP SERVING and is 1.4 CARBS.
How much frosting to use:
Two 8 or 9-inch layers need 4 cups of frosting. (1.8 carbs per person assuming 12 servings.)
Three 8 or 9-inch layers need 5 cups of frosting. (2.3 carbs per person assuming 12 servings.)
Use 1.5 cups of frosting per 12 cupcakes. (2 tbsp per cupcake or .7 carbs)
Use 3 cups of frosting per 24 cupcakes. (2 tbsp per cupcake or .7 carbs)