Preheat oven to 180C (350F/ Gas Mark 4). Grease and flour a 23cm (9-inch) round cake tin.
In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, bicarbonate of soda, and salt.
In a separate bowl, whisk together eggs, milk, oil, and vanilla extract.
Add the wet ingredients to the dry ingredients and mix until just combined.
Gradually add the boiling water, mixing until the batter is smooth (the batter will be quite thin).
Pour the batter into the prepared cake tin and bake for 30-35 minutes, or until a skewer inserted into the center comes out clean.
Let the cake cool in the tin for 10 minutes before transferring it to a wire rack to cool completely.
For the Fudge Icing:
Melt the butter in a saucepan over low heat.
Remove from heat and whisk in the icing sugar and cocoa powder until smooth.
Gradually add the milk and vanilla extract, mixing until the icing is glossy and spreadable.
Once the cake is completely cool, spread the fudge icing evenly over the top.
Slice and serve.
Tips and Substitutions: For a richer flavor, use dark chocolate cocoa powder. Buttermilk can be used in place of regular milk for a tangier flavor. For a gluten-free option, substitute the plain flour with a gluten-free flour blend. Adding a tablespoon of instant coffee to the batter can enhance the chocolate flavor.