Pre-heat oven to 160 degrees Celsius, and grease and line two 20 cm round spring form cake tins.
In a medium bowl add the flour and baking powder, then sift in the bicarb soda and cocoa powder. Whisk to combine then set aside.
In the bowl of an electric mixer, beat the butter, sugar and vanilla for about 2 minutes, until it is light and fluffy.
Beat in the eggs one at a time mixing well after each.
Then add half the flour mixture and gently mix. Then add the milk and crème fraîche and slowly mix until combined (you may want to cover your mixer with a tea towel at first so the milk doesn’t splatter). Then add the remaining flour and mix until just combined.
Evenly divide the mixture between the prepared tins and smooth the tops of each.
Place in the oven on the same rack if possible, and bake for 30-35 minutes, or until the cakes spring back when touched lightly and a skewer comes out clean.
Remove from the oven and leave to cool in their tins for a few minutes before removing from the tins and allowing the cakes to cool completely on a wire rack.
To make the icing, place the milk, water, dark brown sugar, treacle and butter in a medium saucepan and place over medium heat. Stir constantly until the butter is melted and everything is combined.
Turn off the heat and tip in the finely chopped chocolate.
Mix together with a spatula. Then remove from the stove and continue mixing with a whisk until the chocolate has fully melted and is well incorporated.
Then sift in the icing sugar one tablespoon at a time, whisking well after each.
Place the icing in the fridge for about 30 minutes. Whisk every few minutes so it doesn’t firm up too much. The icing will be ready when it has thickened and is a spreadable consistency. If it becomes too thick leave at room temp until you can whisk it again and it is a spreadable consistency.
Once the icing has thickened, measure out 1/2 cup of the icing, and place in a small/medium bowl. Add the cream and sift in the 1 tbsp cocoa powder. Briefly whip with a handheld mixer until everything is combined and the icing has a mousse like texture.
To assemble the cake, start by trimming the tops of each cake so they have a flatter surface. You shouldn’t have to trim too much off.
Place one of the cakes on a serving plate or cake stand (top side up), and spread over the icing and cream mixture you just whipped up. Top with the second cake (top side down).
Evenly spread the remaining icing over the top and sides of the cake, making sure you fill in any gaps that appear in between the two cake halves.