Chop the dates, place in a pan with the sugar, vanilla essence and water and stir. Simmer until the dates have softened and the mixture resembles pulp. Add more water if needed. Leave to cool.
To make the crumble
Sift the flour into a large bowl and add the baking powder and butter. Rub in the butter.
Add the oats and granulated sugar and mix.
To assemble the date and oat slices
Line the tin with non stick baking parchment.
Place half the crumble mixture into the base of the tin and compact with the back of a spoon.
Spread the date filling over the base leaving a small gap around the edge so that the filling does not seep out while it is baking.
Spread the remaining half of the crumble mixture over the top of the date filling and sprinkle a few extra oats on top.
To bake
Bake in the centre of the oven as 190C, 375F, gas mark 5 until golden (around 20 to 30 minutes).
Place on a wire cooling rack and leave to cool in the tin. Cut into squares or rectangles when cold.