Assemble all of the ingredients for the cake & melt the butter; leave it to the side to cool. Preheat the oven to 175C & grease + flour two 7-inch cake tins.
In a large bowl, sift together the sugar, flour, cocoa powder, baking powder, & bicarbonate of soda. Use a whisk to combine the sifted ingredients.
Add the cooled butter, eggs, & the milk to the bowl & beat until smooth. Pour the boiling water into the bowl & beat until smooth. (Yes, mixture will be very liquid-ey, but have faith).
Divide the batter evenly between the two cake tins & bake in the preheated oven for 35-40 minutes, or until a skewer inserted into the centre of the cakes comes out with moist crumbs. Leave the cakes to cool in the tins for 5-minutes, then turn them out onto wire racks to cool completely.
While the cake cools, make the frosting. Place the chocolate into a heatproof bowl in the microwave & melt in 20-second intervals – taking out & stirring with a spatula between every 20-seconds (be very careful not to burn the chocolate, as there is no way to fix this). Once melted, leave to cool slightly.
In a medium bowl, use an electric whisk to beat the butter until light & fluffy. With the mixer on low speed, add in the coffee, icing sugar, and vanilla extract & beat together until creamy. Add the melted chocolate to the mix & beat until lusciously smooth.
When the cakes have completely cooled, cut each layer in half horizontally – giving you four equal thin layers. Place one of the layers on to your cake stand/plate & cover with a layer of frosting + another cake layer. Continue the layering process until the last layer has been added, then cover the entire cake with the remaining frosting, trying to keep the outer layer ass smooth as possible. (I always apply a thin layer of frosting first, leave it in the fridge for 15-minutes, & when this layer has chilled I do my final layer of frosting.) (If you have any leftover frosting, keep aside for additional decoration.) Place the cake in the fridge while you make the chocolate glaze.
Place the butter & the chocolate into a heatproof bowl in the microwave. Met together in bursts of 20-seconds, ensuring to remove from the microwave to stir in between every burst. Once melted, leave aside to cool to room temperature.
Remove the cake from the fridge & evenly pour the chocolate glaze over the surface, using an offset spatula to help push it over the edges. The glaze should drip easily down the sides of the cake. Return the cake to the fridge.
If you have leftover frosting & you decide to decorate the cake further, pop it into a piping bag & squeeze blobs over the top of the glaze. Sprinkle the flaked salt over the top of the cake, if desired.