Preheat the oven to 170C/150C (fan) and line an 8×8″ baking tin with non-stick baking paper
In a large mixing bowl, whisk together the plain flour, caster sugar, cocoa powder, baking powder, bicarbonate of soda, and salt with a balloon whisk
Add the egg, vegetable oil, milk, and vanilla extract and whisk until smooth
Slowly pour in the warm water and mix gently until fully combined
Pour the mix into your prepared baking tin
Bake in the oven for 30-40 minutes or until a toothpick inserted into the centre of the cake comes out clean. Let it cool fully in the tin
For the buttercream icing
In another large mixing bowl, using an electric whisk, beat the butter until creamy
Gradually add the sifted icing sugar, cocoa powder and milk, about a third at a time, whisking until smooth and creamy, until you reach your desired consistency
For the chocolate fudge sauce
In a medium mixing bowl, with a spoon or spatula, stir together the condensed milk and melted dark chocolate until thick and smooth
Assemble the cake
Using the end of a mixing spoon or similar, poke 16 holes 3/4 of the way down into the cake
Pour the chocolate fudge sauce over the cake and into the holes, then smooth it out evenly
Scoop the buttercream icing onto the cake and gently smooth it out to the edges
Top with chocolate shavings and refrigerate for at least 1 hour
Notes
Store in an airtight container, in the fridge for up to 5 days
Make sure to let the cake cool fully in the tin before proceeding to the next steps