In an oven-proof glass bowl placed over a saucepan of boiling water, melt the chocolate and butter together.
Mix the sugar and egg yolks into the melted chocolate mixture. In a separate bowl, beat the egg whites until they form stiff peaks. Gently fold the egg whites into the chocolate mixture. Spoon the combined ingredients into a well-greased, small, deep cake tin or divide among 6 individual oven-proof dishes.
If using a single cake tin, bake the fondant in the preheated oven at 200°C for 20 minutes. If using individual dishes, reduce the baking time to 12 minutes. To prevent the tart from drying out, cover it with foil after 10 minutes of baking the cake pan, or 6minutes if baking the individual dishes. The tart may appear slightly runny when removed from the oven, but it will firm up as it cools. Once cooled, dust with icing sugar and adorn with fresh berries and mint.