Preheat the oven to 350ºF/ 180ºC and line a cupcake tray with 8 cupcake cases. In a bain-marie over medium heat, melt the cocoa mass (or unsweetened dark chocolate if using), set it aside to cool slightly. In a large bowl of a stand mixer with the whisk attachment, whisk the softened butter and erythritol together until light and fluffy, about 5 minutes on medium-high speed. Add in the eggs, vanilla extract and unsweetened almond milk, whisk until smooth and then mix in the melted cocoa mass.
Gather the dry ingredients and place the hazelnut flour, coconut flour, ground linseed, psyllium husk and a pinch of sea salt in a large bowl and stir to combine. Add to the wet ingredients and mix slowly building up to medium speed as it begins to combine. The batter will be thick, but scoopable. If your batter feels too thick just add more almond milk one tablespoon at a time to thin it out.
Spoon the mixture equally into the cases. Use an ice-cream scoop as a guide, about 2 tablespoons per case. Place in the middle of the oven and bake for 15-18 minutes until cooked through. Remove from the oven and leave them to cool on a wire rack.
To make the chocolate buttercream frosting, add the softened butter to the bowl of a stand mixer with the paddle attachment. Beat on high speed for at least 5 minutes. The butter will double in size and become paler in color.
After 5 minutes, give the butter a stir with a spatula, scraping the sides of the bowl and making sure all the butter is equally mixed. Give it another beat for a minute or two. Add the mascarpone and continue beating for a minute until combined. Sift in the cocoa powder and powdered erythritol to the butter. Slowly begin to mix until it’s incorporated and gradually increase the speed to high until well beaten. Scoop the frosting into a piping bag (if using) and place it in the fridge for 10 minutes to firm up slightly. Once the cupcakes have completely cooled, frost each cupcake.