Preheat the oven to 180oC (350oF). Grease a deep 8-inch (20cm) cake tin with oil or butter.
In a medium pot, melt the butter over a low heat until just melted. Remove from the heat and stir in the salt, almond extract and milk. Add the egg, granulated sugar and ground almonds - stir again until well combined. Add the flour and baking powder and stir together until just combined.
Pour the batter into the prepared cake tin. Cut the rhubarb into lengths roughly 7-10cm (3-4 inches) long . Use them to decorate the top of the cake in a radial pattern. Sprinkle the flaked almonds around the edge of the cake. Sprinkle the whole surface of the cake with 1-2 tbsp of granulated sugar.
Bake for 40-50 minutes until well risen and a skewer inserted into the centre of the cake comes out clean (the surface of the centre may still look a bit gooey due to the moisture from the rhubarb but will be cooked underneath). Leave to cool in its tin on a wire rack for at least 10 minutes before slicing and serving.