Method (base)

1. Preheat your oven to 180ºC fan and line an 8 x 8 inch square cake tin with greaseproof paper. Allow the paper to hang over the tin slightly, this makes the cake easier to remove when baked.
2. Add the dairy-free butter into a small saucepan, place on the hob and heat over low until fully melted. Remove from the heat and add into a medium sized, heat-proof mixing bowl.
3. Add the biscuits / graham crackers into a blender / food processor and blend until fine. Pour the biscuit crumbs into the melted butter and stir together using a wooden spoon or spatula. It should resemble wet sand, and when you press it between your fingers, should hold its shape. Press the mixture into the lined tin, compacting it down into an even layer. Set aside. 

Method (Cake)

1. In a bowl, combine the dairy-free milk with the apple cider vinegar and yogurt then whisk until fully combined. Set aside for 10 minutes to curdle. This creates a vegan ‘buttermilk’.
2. In a large mixing bowl, sift the sugar, flour, cocoa powder, baking powder and bicarbonate of soda. Mix well to combine. Add the oil into the ‘buttermilk’ and whisk to combine.
3. Add the wet ingredients into the dry ingredients and mix just to combine.
4. Spoon the batter into the lined tin over the buttery base. Level it out with the back of a spoon or offset spatula.
5. Place the cake into the middle of the preheated oven and bake for around 25-30 minutes. You will know it is done when you put a knife or skewer down the middle of the cake and it comes out clean and it’s springy to the touch. Check 25 minutes, its better to slightly underbake a chocolate cake to keep it moist than to over-bake as over-baking will dry the cake out. If batter comes off on the skewer, it needs more time to bake. To prevent burning, top the cake with some greaseproof paper then return back into the oven. 
6. Allow the cake to cool in the tin for 10 minutes then carefully transfer onto a cooling rack. Allow to cool fully before filling and frosting. You can even place the cake in a sealed container once cool then frost the day after. It’s a very light sponge so be careful with it when removing from the tin.
7. Optional: Add a simple sugar syrup over the cake before storing to add some extra moisture into the cake. Mix equal parts fine white sugar and water in a saucepan on the hob and heat until the sugar has dissolved. Allow this to cool then brush over the top of the cake.
8. Once the cake has fully cooled, poke holes into the cake, transfer some chocolate fudge sauce (or vegan chocolate ganache) into a piping bag and pipe in the holes. This will create hidden pools of chocolate in the cake. 

Method (buttercream)

1. Add the dairy-free dark chocolate in a heat proof bowl, place on top of a saucepan filled 3/4 with water and place on the hob. Melt the chocolate, then leave it to cool slightly.
2. Beat the dairy-free butter until light, smooth and creamy. Add in the vegan sour cream and mix until fully combined.
3. Gradually add the icing sugar and cocoa powder, beating until smooth.
4. Slowly pour in the melted chocolate and beat until the buttercream is silky, glossy and fluffy.
5. If it’s too thick, add a small splash of dairy-free milk. If it’s too soft, chill for a few minutes before using.
6. Spread the chocolate buttercream on top of the cake, using a spoon or off-set spatula to spread it out (refer to video for example). 

Method (marshmallow fluff)

1. Ensure your mixing bowl and whisk is clean and free from any grease / moisture.
2. Add the aquafaba, vanilla bean paste, and cream of tartar into a large bowl or the bowl of a stand mixer. Use a balloon whisk attachment on your stand mixer or a hand mixer with whisks to beat the liquid for around 1 minute until it starts to get fluffy.
3. Slowly sprinkle in the sugar into the mixture, a tablespoon at a time while the mixer is on. This will prevent deflating the meringue. Continue to beat for about 10-15 minutes until stiff peaks form. The meringue will be thick, glossy and when you lift the whisk up.. it should stay as it is. Depending on your mixer, the speeds and temperature in your kitchen, it may take longer to get to the right consistency.
4. Once thick, spoon the marshmallow fluff on top of the chocolate buttercream. Use an off-set spatula or back of a spoon to swirl the meringue out.
5. Torch the meringue using a kitchen blow torch, being very careful to not over torch / burn the meringue.
6. Use a shark knife to cut the cake into squares, serve and enjoy!