This chocolate cake is a perfect dessert option when a guest needs to avoid gluten: No one else at the party will have a clue that it's gluten-free! My secret weapon is Thomas Keller's "Cup4Cup" gluten-free flour. I substitute it, literally cup for cup, for the flour in most recipes, and trust me, no one can tell the difference! Happy guests and a happy host!
Preheat the oven to 425 degrees F. Grease an 8-inch springform pan and line the bottom with parchment paper.
Place the one pound of chocolate in a large heatproof 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, coffee granules and salt with a rubber spatula. Whisk in the egg yolks until smooth.
Place the egg whites in the bowl of an electric mixer fitted with the whisk attachment 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. (Be sure your oven temperature is accurate!)
Turn the oven off and leave the cake in the oven, leaving the door slightly ajar. Allow the cake to cool in the oven for 1 hour. The cake will sink in the middle.
Carefully release the sides of the pan and slide the cake onto a flat serving plate or cake stand. Place the 3 ounces of chocolate and the heavy cream in a heatproof bowl set over a pan of simmering water and heat just until the chocolate melts, stirring occasionally. Add a drop of cream if the mixture is too thick to pour. Pour the chocolate onto the sunken part of the cake, leaving the edge unfrosted. Allow to cool and serve warm or at room temperature with a scoop of ice cream.
You can also make this cake with one tablespoon all-purpose flour instead of gluten-free flour.