Preheat oven to 180ºC/ 350ºF. Grease a 9-inch/ 24 cm round cake pan with nonstick cooking spay or butter. Set aside.
Break chocolate (leaving a couple os squares in the fridge to spread over cake at the end), add the butter and place into a heatproof bowl. Sit over a pan of barely simmering water (a bain marie) and allow the chocolate to melt, stirring occasionally. Once melted, remove from heat and cool to room temperature.
Separate the egg yorks from the whites. Beat sugar and egg yolks together in a large bowl with an electric mixer until light and fluffy. Slowly stir in chocolate mixture until just combined.
Beat egg whites in a separate bowl with an electric mixer until stiff peaks form.
Using a rubber spatula, add the beaten egg whites onto the chocolate mixture. Gently run spatula around the edges of the mixture lifting the bottom up and over the center, repeating until well incorporated. Fold until batter is smooth.
Refrigerate 1/3 of the batter and pour the rest (2/3) into a 9-inch/ 24 cm round cake pan. Bake in the preheated oven until edges are puffed and surface is firm, 10 to 15 minutes.
Allow cake to cool to room temperature, about 30 minutes. Spread the refrigerated chocolate mousse evenly over the cooled cake and let it set up in the refrigerator for a few hours before cutting and serving.
Cut the cake into slices and serve with thin shavings from leftover chocolate chunks (tip: use a grater). This cake is also great with powered sugar or raspberries.
Low FODMAP safe serving is 1/7 of the cake: according to Monash University we can have 1 oz / 30 gr. of dark chocolate per meal. The recipe calls for 7 oz / 200 gr.