Place the butter in a saucepan and melt over medium heat. Meanwhile, in a large mixing bowl or sealable plastic bag, tip in all the Biscoff biscuits and crush them with a heavy saucepan or mug or rolling pin until they are crumbs. Combine the biscuit crumbs and melted butter in a mixing bowl along with the pinch of salt so it is all nicely coated in the butter.
Fill the tart tin with the crumbs and press it across the base and up the side with the back of a spoon, spreading it out and flattening it. Chill the base in the fridge for at least 15 minutes.
Now for the chocolate mousse - melt the chocolate in a bain-marie over barely simmering water. Combine the butter and sugar in a large mixing bowl and mix with electric beaters (or in a stand mixer) for at least 5 minutes, until the mixture is creamy, soft and fluffy, and the sugar has dissolved into the butter.
Pour the melted chocolate into the butter and sugar mixture, then crack in one egg. Scrape the sides of the bowl then whisk the mixture again for another 5 minutes - it will gradually lighten and become creamier. Add the other egg and beat again for another five minutes, until it is light, smooth and creamy, like mousse.
Take the Biscoff base from the fridge and fill it with the chocolate mousse, spreading it out to the edges and smoothing the surface. Chill for 30 minutes to 1 hour until firm to the touch.
Combine the caramel with the Biscoff spread in a small bowl. When the mousse is ready and firm, spread the caramel on top - it will be quite thick so dollop spoonfuls all over the surface then gently spread them together. Return it to the fridge until ready to serve.
When it is time to eat, slice the bananas into rounds and lay them on top of the caramel. Whip the cream with the optional spoonful of icing sugar until it reaches medium peaks - soft waves of cream that aren't too stiff. Pile it all on top of the tart, then grate over a little extra dark chocolate before serving.