First, make the jam. Heat the berries, water and honey together until the mixture starts to boil. Cook for around five minutes, add the cornflour, and then cook for a further minute or two until it starts to thicken. Remove from the heat and set to one side.
Pre-heat the oven to 175ºC (350ºF). Line a large baking tray with grease-proof (baking) paper.
Beat the butter and sugar together until nice and fluffy. Add the egg (you may need a whisk to get it all nice and combined).
In a separate bowl, mix the flour, baking powder and vanilla sugar and add to the wet ingredients. Stir until a dough forms.
Split the dough into three fairly equal parts. Roll each one into a long, thick sausage and flatten down slightly (it should be about 35cm long). Take a small spoon or your thumb, and gently make an indentation all the way down the roll. Spoon the cooled jam along the indentation. Repeat with the other two bits of dough.
Bake for around 15 minutes. The cookies will have started to brown but will still be pale. Remove from the oven and allow to cool completely.
Mix one tablespoon of water with enough icing sugar to make a thick drizzle. Dribble over the baked cookies, slice diagonally and serve. Enjoy!