Wash and chop the rhubarb into 1 inch slices.
Transfer in a pan, with sugar, vanilla extract and amaretto liqueur. Cover with a lid and cook until the rhubarb is soft but with still a bit of a bite (max 5 mins). Set aside to cool.
Preheat the oven at 200°C (390°F).
For the crumble add in a bowl the flour, the brown sugar, the ground almonds and the vegan margarine (soften at room temperature). Mix everything together with the finger tips to a coarse sand like texture. Add the almond shavings, the almond essence and baking powder, and incorporate well to the mixture.
Pour the cooked rhubarb in a 18-20 cm (7-8 inch) round ceramic dish.
Evenly spread the amaretto crumble over and bake for 25-30 minutes or until golden brown.
Serve the vegan rhubarb & amaretto crumble warm with icing sugar on top and some silky smooth vanilla custard.