Preheat oven to 170 C or 150 C fan. Grease a 20cm square tin and line the base with baking paper.
Sift flour , cocoa powder and sugar into a large mixing bowl and make a well in the centre.
Whisk milk, eggs and vanilla together in a jug and add to the dry ingredients, along with the butter. Fold together until just combined, but don't mix or beat. Pour into prepared pan. Bake for 50-55 minutes, until a skewer comes out clean when inserted into the centre. Cool in the tin for 10 mins, then turn out onto a wire rack to cool completely.
For the topping , place cream into a small saucepan and bring to the boil. Add chocolate and turn off the heat. Stand for 5 mins, until chocolate is melted and smooth. Refrigerate mixture for about 20 mins, stirring occasionally , until thickened. Spread over top.