Pre-heat oven to 350*F and grease then flour two 9-inch cake pans (I used coconut flour to flour the pans, you can also use two 8-inch pans).
Add your 18% cream to a measuring cup along with 1 tsp of vinegar and set aside.
In a large bowl, beat together your butter and sweetener until light and fluffy. (Make sure to partially stir in your sweetener before beating it to prevent it from flying everywhere).
Next add in your vanilla extract and your 6 egg whites and beat until well-incorporated.
In another medium bowl, whisk together your almond flour, coconut flour, baking powder, xanthan gum and salt.
Add these dry ingredients along with your vinegar/cream mixture and beat until everything is incorporated (try not to over-mix the batter).
Finally gently fold in your sprinkles and then divide the batter between the two cake pans, using a spatula to smooth out the tops as best as possible.
Bake for 40min or until the cakes have golden brown edges and the centres are not wet to the touch (note that these will set-up more as they cool, keep an eye on the cakes as my oven tends to bake on the slow side).
Cool the cakes on a wire rack for 15-20min before gently flipping them onto plates to remove and cool further. While the cakes are cooling, you can prepare the frosting.
Whip your butter and cream cheese together using a kitchen aid stand mixer with the whisk attachment or a hand held mixer until light and fluffy, scraping down the sides occasionally.
Add your sweetener, salt and vanilla extract and beat again, starting very slow then increasing the speed to prevent the sweetener from flying everywhere. Beat until smooth.
Spread a thick layer of frosting over the top of one cake, reserving about 2/3 of frosting for the rest of the cake.
Stack your 2nd cake on top of the first one (if you get a crack don't worry, just put it back together and ice it! No one will know, that's the beauty of frosting). Ice the top layer as well as the sides of the whole cake now until fully covered.
Add sprinkles or writing using additional frosting (see tips) and a piping bag and then serve! Enjoy your Keto Funfetti Birthday Cake! This is also great chilled if you have the time to place it in the fridge overnight!