Melt the vegan butter. In the meantime crumble the cookies (filling included, since it would give an extra stickiness and vanilla flavour to the base).
Combine cookies and melted butter and arrange on the bottom and sides of the tart. Note: If not using a loose bottom tin, make sure you coat the tray with some film to remove the tart once it has set.
Place in the freezer for at least 15 minutes.
Heat the plantbased milk on a stove. Stir into the Biscoff spread until creamy then add the agar agar. Heat the mixture for a couple of minutes after it starts to bubble (stirring constantly).
Pour the Biscoff mousse over the base, spread evenly then pop into the frigde once again.
In the meantime prepare the chocolate mousse. Melt the dark chocolate either via bain-marie or in the microwawe (for 2 minutes at 350-400W, stirring halfway). Add vanilla extract.
Whisk the aquafaba until it forms stiff peaks, then gently combine it with the melted chocolate, one spoonful at the time.
Pour the mousse on top of the Biscoff spread layer and let it set in the freezer for at least 2 hours before decorating/serving.