Preheat the oven to 355* and line a muffin tray with 10 cases.
Stir together the milk and lemon juice in a mixing bowl and leave for 5 minutes to curdle and form a “buttermilk”.
Pour in the coffee, sugar and oil and whisk.
Sift in the flour, cocoa powder and baking soda and add the salt. Whisk to a smooth batter with no specks of flour.
Use an ice cream scoop or large tablespoon to divide the batter between the cases, filling them about 2/3 full.
Bake for 17-20 minutes, until springy to touch and an inserted skewer comes out clean. Remove from the tins and cool fully on a wire rack.
Core the centre from the cooled cupcakes and keep these bits. Warm the chocolate spread for the filling.
Add the butter to a mixing bowl and beat with a handheld electric whisk or freestanding mixer until fluffy. Gradually add I the vanilla, icing sugar and aquafaba or plant-based milk and beat for 2-3 minutes until really fluffy. Transfer to a piping bag with a round nozzle attached.
Melt the chocolate and coconut oil together. Pour into a glass that’s wide enough to dip in the cupcakes.
Fill the cores of the cupcakes with the warm chocolate spread and add on the top piece of cake. Pipe on the vanilla buttercream and dip the cupcakes into the glass of chocolate carefully and quickly, allowing the excess to dip off while turning the cupcakes (don’t leave them upside down too long or the frosting will drop off).
Stand the cupcakes up and sprinkle with crushed hazelnuts. Eat straight away or allow the chocolate to set. Store the cupcakes in the fridge in a sealed container for 2-3 days. Unfrosted and unfilled cakes will keep for up to 1 month in the freezer.