Preheat the oven to Fan 170C/Gas 4. Grease a 30 x 20cm brownie, cake or shallow roasting tin with butter and line the base with non-stick baking paper.
Put the butter, 175g sugar, eggs, almond extract, ground almonds, flour and baking powder in large bowl. Beat with a wooden spoon until smooth and thick (You can also do this in a mixer or food processor).
Spoon the cake batter into the prepared tin and spread to the sides – it will be fairly thick. Top with the rhubarb, pressing gently into the cake batter.
Sprinkle with the reserved sugar and then the flaked almonds.Bake for about 30-40 minutes, or until the cake is golden, risen and firm. If you insert a skewer into a couple of places in the centre of the cake, avoiding any pieces of rhubarb, it should come out clean.
Serve warm or cold, cut into squares. This cake keeps well for up to three days in the fridge and can be frozen wrapped tightly in foil for up to a month.