Preheat the oven to 180°C (360°F).
Crumble the amaretti biscuits to a fine crumb (I whizzed them briefly in a food processor). Add the buckwheat flour, rolled oats, ground almonds and brown sugar and stir to combine. Grate in the cold butter and mix it in with your hands. Set aside.
Drain the cherries, reserving the juice. Place cherry juice, caster sugar, vanilla pod and seeds in a saucepan over medium heat, stirring to dissolve sugar. Dissolve arrowroot in 2 tablespoons of the warm cherry juice mixture, then return to the pan, whisking to combine. Cook, stirring, for 6-8 minutes until thickened, then add the cherries and stir gently to combine. Take off the heat and add the amaretto liqueur, if using (I suggest you start with one tablespoon, taste, then add more if you want to).
Divide cherry mixture among six 1-cup (250ml) ramekins. Let the cherries cool a little, then cover generously with crumb mixture (you’ll have quite a bit left over which you can store in the fridge for another time). Dot the finely diced butter evenly over top of the crumble.
Bake for 15-20 minutes until the topping is golden and the juices are bubbling. Once baked, set ramekins on a wire rack to cool. You can either serve the puddings while still warm or, as I prefer, store in the fridge to chill. Take out of the fridge about 15 minutes prior to serving and dollop some cream on top.