Yesterday, I made a crumble with buckwheat flour. I wouldn't recommend it. The crumble was perfectly pleasant, but somewhat powdery -- which is not the quality you want. But the plums -- a blue-black variety called Swenson's -- were good. I cut them in half, stoned them, and placed them in my oven dish, before sprinkling them with a dstsp of caster sugar and vanilla essence. (Vanilla essence is not an easy substance to sprinkle; but I did my best.)
There were only two of us. I used in all 12 plums, 70g of flour, 30g of butter, 2dstsps of sugar, 2tsps of vanilla essence, and more butter for the top of the crumble. I cut the butter into small pieces, and put it back into the fridge for 30 minutes before whizzing it in the food processor just until it and the flour had combined. I stirred in my second dstsp of caster sugar, scattered the crumble mixture on top of the plums, and distributed slivers of butter on top. I baked the crumble for 35 minutes at gas mark 5/190C.