Preheat the oven to 425° F.
Grease an 8-inch spring-form pan and line the bottom with parchment paper.
Place the one pound of chocolate in a large heat-proof bowl set over a pan of simmering water and stir occasionally until the chocolate is melted.
Off the heat, immediately stir in the butter, flour, sugar, and salt with a rubber spatula.
Whisk in the egg yolks until smooth.
Put egg whites in mixer and beat until they form soft peaks.
Scrape the egg whites into the chocolate mixture and fold them in very carefully with a rubber spatula, just until combined.
Scrape the mixture into the prepared pan, smooth the top, and bake for 15 minutes exactly. (Make sure oven is accurate-next time I will bake it 13 minutes!)
Turn the oven off and leave the cake in the oven, cracking the door open a bit.
Let cake cool in the oven for 1 hour.
The cake will sink in the middle.
Remove sides of pan and place cake on your cake stand.
Put 3 ounces of chocolate and the heavy cream in a microwave safe bowl and heat gently until it is melted and smooth.
Pour this into the center of the cake.
Allow to cool and serve warm or at room temperature with a scoop of ice cream. I stored mine in the fridge. We liked it better cold.