1. Preheat the oven to 350°F (180°C). Grease two 8-inch (20 cm) round cake pans and line the bottoms with parchment paper.
2. Place the dates, chocolate and baking soda in a heatproof bowl. Pour the boiling water over the dates and let sit for 10 to 15 minutes to soak. Pour everything, including the liquid, into a food processor (or use an immersion blender) and purée until smooth.
3. Cream the butter and brown sugar together by hand until well combined. Add the eggs one at a time mixing well after each addition (you may wish to switch to a whisk). Add the Medjool date mixture and whisk in.
4. Sift the flour, cocoa powder, cinnamon, baking powder, salt and cayenne into the bowl and whisk until well combined. Divide the batter between the two pans and tap the pans. Bake for about 35 minutes until a tester inserted in the center of a cake comes out clean. Cool the cakes in their pans on a cooling rack for 15 minutes, then tip the cakes out of the pan to cool completely.
5. For the frosting, beat the butter and icing sugar together using electric beaters or a stand mixer fitted with the paddle attachment until fluffy and smooth, about 3 minutes. Add the cream cheese and vanilla and beat on medium speed for another 3 minutes until fluffy and the frosting holds its shape well.
6. To assemble the cakes, place one cake layer on to a plate or platter and spread or pipe half of the frosting on to the top of the cake. Place the second cake layer on top and repeat with the remaining frosting. Chill the cake uncovered until ready to serve.
The cake will keep, loosely wrapped and refrigerated for up to 3 days.
*While Medjool dates are high in natural sugars, they are low on the glycemic index thanks to a balanced amount of soluble fiber. The complex carbohydrates found in Medjool dates are digested at a slower rate, making them a source of dietary fiber and releasing energy to the body over a longer period of time.