Heat the oven to 180ºc / 160ºc fan and line three 20cm/8inch cake tins with baking parchment
In a stand mixer, beat together the butter and light brown sugar until light and fluffy.
Add in the flour, cocoa powder, beaten eggs, baking powder and beat again briefly till combined – try not to over beat the mixture!
Divide the mixture between the three tins and smooth it over – bake for 25-30 minutes until a skewer comes out clean when poked, and when the cake springs back.
Once baked, leave the cake to cool in the tin for 10 minutes, and then remove and leave to cool fully on a wire rack. If the cake has domed slightly, leave the cake to cool upside down to flatten it slightly.
Frosting
In a stand mixer, beat the room temperature butter until it is smooth
Beat in the icing sugar 1/3 at a time until its fully combined.
Keep beating the buttercream for a few minutes so it starts to get fluffier and lighter.
Add in the tub of marshmallow fluff and beat again – until combined
Decoration
Once the cakes are cooled, put the first layer on the serving plate spread 2-3tbsps of buttercream onto the top of the first layer
Repeat with the second cake on top, and then the final sponge layer.
Using a bit more of the buttercream, add a thin layer around the edges of the cake and scrape to smooth - refrigerate for 10-30 minutes to set
Repeat again with a slightly thicker layer of buttercream. I slather it on all over using an offset spatula and then run the metal scraper round until its smooth.
Refrigerate the cake for about 20 minutes to firm it up.
Once finished, melt the dark chocolate in a heatproof bowl until melted.
Add in 1tbsp of oil and beat until smooth, continue to add oil till you get to a drippy consistency. I use 2tbsps of oil for dark chocolate drips, but some chocolates might need more or less.
Carefully pipe it around the edge of the cake, edging over slight bits to create the drip. You don’t need to use too much per drip as it’ll drop quite far down by itself. Fill in the top in with the rest of the chocolate so the top is also covered.
Add some larger marshmallows around the edge of the top of the cake and toast them slightly with a blowtorch for a bonfire marshmallow effect. Crush over some biscuits and leave the chocolate to finish setting.