Grease an 8X8 square metal cake pan. Set a rack to the middle of the oven and heat to 350℉.
To make the cake, mix granulated erythritol, Keto Chow, and almond flour together in a small bowl and set aside.
Pour the hot coffee and cocoa powder into a large mixing bowl and stir together until smooth.
Add the oil and sour cream to the coffee and cocoa and mix well. Then add the baking soda and salt.
Add eggs and vanilla extract and mix well.
Pour the dry ingredients into the wet and beat until the batter is smooth.
Pour the batter into the prepared cake pan and bake on the middle rack of the oven for 20 minutes, or until the edges are firm and the center is set. It may still look slightly wet in the center, but the tip of a knife or skewer should come out clean when inserted.
Cool the cake completely, then place in the refrigerator to chill. The cake will continue to firm as it chills.
To make the buttercream, beat the butter in a large mixing bowl until light and aerated.
Add the powdered erythritol and Keto Chow and beat until completely combined.
Add the heavy cream and vanilla and beat until smooth and creamy. The mixture should remain slightly stiff. Transfer buttercream to a piping bag fitted with a large round piping tip. If the buttercream isn’t stiff enough to hold its shape when piped, place in the refrigerator to chill for about ten minutes.
Pipe six evenly spaced lines across the cooled cake from one edge to the other. There should be about an inch between each line of buttercream. If the bumps aren’t completely even, you can smooth them gently with a warm metal spatula.
Place the cake in the freezer to set the buttercream.
To make the glaze, place half the butter with the allulose, cocoa powder, almond milk, yacon syrup, and salt in a small saucepan and bring to a simmer over medium heat, stirring continuously.
Let simmer until it begins to thicken, bubbles that are slow to burst cover the surface, and it leaves a visible trail that doesn’t close back up right away when the spoon is drawn through it.
Remove the pan from the heat and add the remaining butter in chunks. Stir until the butter has melted.
Add the powdered erythritol and vanilla extract and mix until the mixture is thick and glossy.
Allow the mixture to cool slightly, then remove the cake from the freezer.
Pour the glaze over the buttercream lines first, covering them completely. Place the cake back in the freezer for a minute or two to allow the glaze to set.
Once set, pour the rest of the glazebetween the buttercream lines, covering the entire cake. It should fill most of the space between the buttercream, leaving just a slight bump at the top to give its signature bumpy look.
Chill again for at least an hour, or until the glaze is set.
Store finished cake tightly wrapped in the refrigerator for up to a week or in the freezer for up to three months.