Preheat the oven to 170deg fan assisted. Grease and line the base of a shallow 29cm x 19 cm oblong tin or similar, at least 2cm/1ins deep, with baking parchment. This will help you to lift the cake out a lot easier once it's baked and the topping has set.
Using an electric hand mixer, beat together the butter and sugar until light and fluffy. (Fig1 in post). Add the eggs one at a time adding a little flour in between each one. This will help prevent the mixture curdling. Mix until well combined. Scrape down the sides of the bowl as you go along.
Add the Vanilla Extract, cocoa powder and remaining flour. Mix together until well combined.
Pour the cake mixture evenly into the prepared tin. (Fig 2 in post). Bake for 25 -30 mins in the centre of the oven or until baked. Testing the centre of the cake, in different areas, with a skewer until it comes out clean.
Leave in the tin to cool completely.
Slice off any domed part of the cake that has risen too high. This is to give you a reasonably leveled surface for all the lovely chocolatey topping.
Return the cake to the same baking tin.
Make the Topping
Melt the chocolate and butter in a heat proof bowl over a pan of simmering water. (Fig 1 in post). Stir frequently until melted, glossy and smooth. (Fig 2 in post. Leave to one side to cool a little.
Pour the chocolate over the top of the cake while still in the baking tin. (Fig 3 in post). This is to help prevent the chocolate from oozing over the sides of your cake.
Before it sets completely, use a fork to create a zig zag pattern in the chocolate.
Score the chocolate topping into individual sections to make it easier to slice when ready to eat.
Omit the butter if you want a firm chocolate topping. Melt the chocolate as mentioned above.
Make sure your chocolate is of good quality. By that, I mean a good high percentage of cocoa solids. I used 70% as it gives a really rich dark chocolatey flavour. Take care with some cheaper quality chocolate as they may not melt very well and could split when mixed with the butter.
Will keep in an airtight container for a few days.