- 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.