Combine the kumquats, lemon juice, lemon zest, allulose, stevia, and water in a small pot. Stir and let sit for at least two hours.

Heat the pot over medium heat until simmering. Simmer, stirring occasionally for about 40 minutes, or until thickened and the mixture coats the back of a spoon. Select at least 1/4 cup of sliced fruit to top the cake with and set aside on a piece of parchment paper. Let cool completely and refrigerate for up to 2 weeks or freeze for several months.

Preheat the oven to 325ºF and line two 8-inch wide cake pans with parchment paper (skip this step if using silicon baking pans). Set aside.

Melt the chocolate and butter in a large bowl (either microwave at 50% power, stirring often until smooth or stir over a double boiler in a saucepan on the stove). Let cool to room temperature, about 20 minutes and then stir in the vanilla and espresso powder.

Using a mixer, beat the whole eggs until they are light yellow and doubled in volume, about 5 minutes. Slowly beat in the allulose, stevia, and salt. Gently fold the cooled chocolate mixture into the eggs until only a few streaks of unincorporated egg remain.

Combine the almond flour, lupin flour, and xanthan gum in a bowl and whisk to combine. Using the same whisk, gently fold half this mixture into the chocolate egg mix until mostly incorporated. Add the second half and fold again until evenly mixed.

Divide between the prepared pans, smooth the tops and bake for 15 minutes, or until just set and a toothpick inserted into the middle comes out clean. Cool for 20 minutes before removing from the pans.

Place one cake round onto a piece of cardboard. Top it with 1/2 cup of the kumquat jam. Invert the second cake round so that the cleanest edge is facing up and place it onto the jam covered round. Set aside.

Melt the chocolate and cream together (either microwave at 50% power, stirring often until smooth or stir over a double boiler in a saucepan on the stove). Stir until completely smooth.

Transfer the cake on the cardboard to a wire rack in a rimmed baking sheet (I like to put a layer of plastic wrap in the bottom to collect any excess chocolate).

Pour the chocolate glaze into the middle of the cake and then use a spatula to spread it evenly over the top and down the sides so that it is evenly coated.

Press sliced almonds onto the sides, if using and then top with the reserved kumquat slices. Refrigerate until set, 1-2 hours before slicing and serving.