In a large bowl, whisk together the sesame flour, sweetener, cocoa powder, ground flaxseed, baking powder, baking soda, espresso powder, and xanthan gum.
Place the mayonnaise, eggs, and kahlua in the bowl of the stand mixer or a large bowl if using an electric hand mixer and mix until combined. Pour half of the dry ingredients and mix until fully incorporated. Add the water and mix until combined. Add the remaining dry ingredients and mix until fully incorporated.
Evenly spread the batter (it will be thick) in the prepared pans. Bake for 25 minutes or until a toothpick inserted in the center of a cake comes out clean. Allow to cool completely before frosting.
Kahlua Coffee Buttercream Frosting
Place the butter and sweetener into a stand mixer and blend until combined. Set aside.
In a small bowl, whisk the remaining ingredients together. If you have a frother, use that instead as it will help the coffee granules absorb better into the cream. With the mixer on low speed, slowly pour in the coffee mixture and blend until fully incorporated.
Once the cakes are cool, remove one from the pan and place it on a serving plate. Spread one quarter of the frosting on the top of the cake. Place a second cake layer on top of that and repeat with another one quarter of the frosting over the top. Add the final cake layer and spread another one quarter of the frosting over the top. Split remaining frosting to decorate the sides and add to a cake decorating tool using the star tip if desired. Optional: shaved dark chocolate over the top.