Make the cake ahead of time: Preheat the oven to 350ºF. Line an 8-inch square pan with parchment paper or spray with nonstick spray. In a coffee grinder, grind the coconut sugar until powdery. Transfer to a small bowl and whisk in the milk, oil, vinegar, vanilla, coffee, and stevia. In a medium bowl, sift the flour, cocoa powder, baking powder, xanthan gum, and salt. Whisk to combine. Add the wet ingredients and whisk just to moisten—do not over-mix! Pour the batter into the pan and smooth the top. Bake in the preheated oven for 10-15 minutes until a tester comes out clean, rotating the pan about halfway through. Allow to cool for 10 minutes in the pan before turning out onto a rack to cool completely. When ready to use, cut into small cubes.
Make the mousse ahead of time: Place all ingredients except xanthan gum in a high-powered blender (such as a Vita Mix) and blend until silky smooth. Add the xanthan gum and blend again. Refrigerate until ready to use, at least 6 hours.
Make the cherry filling: Place the cherries, coconut sugar, and water in a medium saucepan over medium-low heat. Allow to cook, stirring frequently, until the cherries give off enough juice to cover the bottom of the pot and prevent scorching. Add the lemon juice and almond extract. Continue to cook, stirring occasionally, until the cherries are completely cooked and soft, 20-25 minutes (there will still be quite a bit of juice in the bottom of the pot). Turn the heat to low. Remove 3-4 tablespoons of the cherry liquid to a small bowl and allow to cool down for a couple of minutes. Add the cornstarch to the bowl and stir vigorously until combined and there are no lumps remaining. Slowly pour this liquid back into the pot, stirring constantly, until the cherry filling thickens, about 1 minute. Turn off the heat and allow to cool to room temperature.
To assemble the trifle: In a decorative bowl or 6 small individual serving bowls, spoon a thin layer of mousse. Top with a layer of cherry filling, a layer of cubed cake squares, more mousse, and more cherry filling. If desired, top the entire thing with coconut whipped cream. Chill until ready to serve, at least 4 hours.