Finely chop the Dark Cooking Chocolate and place it in a heat proof bowl along with the Butter. Melt until smooth, either in 30 seconds increments in the microwave or over a double boiler. Set aside to cool down.
In a separate bowl, whisk together the Eggs and Brown Sugar for about a minute. Mix in the Melted Chocolate and Butter (see note 1).
Add the sifted Flour and whisk until just combined.
Pour the cake batter into two ramekins, about 8 cm / 3 inch wide (see note 2).
Press the chunk of Dark Cooking Chocolate in the centre of each cake, making sure it is covered by the batter.
Bake the chocolate fondant cakes for 12 minutes (see note 3). Take out of the oven, leave to cool down slightly for 5 minutes then serve straight away while still warm.
Video
Notes
Make sure the melted chocolate & butter isn't too hot anymore or you risk cooking the eggs with the residual heat.
If you use a much smaller or larger ramekin, you will need to adjust the baking time. Since we are serving the cakes in the ramekins, there is no need to grease or flour them. If you want to serve these cakes on a plate instead of in ramekins, you can bake them in small pastry rings that are lined with baking paper and placed over a flat baking tray. You will need to be very careful when transferring the cakes over the plate before removing the rings as they are very delicate.
Note that the baking time could vary depending on your oven and the size of your ramekins. Start checking your oven at around 10 minutes; you don't want to over-bake them!