Chop the rhubarb into 1 inch pieces. Place in to a deep pan and add 50g of sugar and ginger. Cook gently for 10 to 12 minutes until tender, stirring occasionally. Put into a baking dish. Preheat the oven at 180C/350F/gas 4. Sift flour and baking powder. Stir in the remaining 50g of sugar then rub in the butter until the mixture resembles breadcrumbs. Put the crumble mixture on top of the fruit. Bake for 35 to 45 minutes until golden brown. Ice cream: Put the sugar, water, ginger and vanilla essence into a saucepan and dissolve over a gentle heat. When the sugar has dissolved bring the syrup up to boiling point and boil to the thread. To test the sugar syrup dip your finger into cold water then into a teaspoon of the hot syrup which should form threads between your thumb and forefinger when they are drawn apart. Cook the rhubarb in the sugar syrup until tender and then drain. Put the mixture through a sieve and put aside. Whisk the eggs and sugar and add 2 drops of vanilla essence. Add a little boiled cream to the eggs and stir. Keep adding a little at a time until all the cream is incorporated. Place back into a saucepan and place on the heat. Stir until the cream thickens and coats the back of a spoon. Add the rhubarb mixture with the custard mixture and place in a flat dish. Put in the freezer for half an hour. Take out and blend to break up the crystals in the ice cream. Do this twice and then your ice cream will be ready.