In a bowl, combine the flour, cocoa powder, baking soda and fleur de sel. Set aside.
In another bowl, cream the butter, brown sugar and sugar with an electric mixer. Add the eggs and beat until smooth. At low speed or using a wooden spoon, stir in the dry ingredients and chocolate. Cover the dough and refrigerate for 1 hour.
Place the chocolate and cream in a bowl and melt for 1 minute in the microwave. With a spatula, stir until the ganache is smooth. Set aside.
With the rack in the middle position, preheat the oven to 375°F (190°C).
Divide and shape the cookie dough into 8 discs. Press half of the discs into four 5-inch (13 cm) mini cast iron skillets (or ramekins) with sides 1 1/4 inches (3.5 cm) high.
Spoon the ganache over the dough, leaving a 1/2-inch (1.5 cm) border. Flatten the remaining discs to the size of the skillet and place over the ganache, sealing the edges by pressing them together. Freeze at this stage, if desired.
Bake for 12 to 15 minutes (25 minutes if frozen) or until the cookies are baked but still soft in the centre. Let cool on a wire rack. Top with ice cream, if desired.