Preheat the oven to 180°C / 350°F / Gas mark 4
Grease and line a 20 x 30cm baking tin with greaseproof paper or baking parchment.
Whisk the butter, caster sugar, vanilla and egg yolk together in a large bowl until pale.
Stir in the flour with a pinch of salt using a wooden spoon then your hands to combine everything together into a crumbly dough.
Press half into the base of the tin evenly and smooth down the top.
Spread the jam over, leaving a 1cm empty border all around the edges.
Crumble over the remaining mixture, then pat down, being careful not to disturb too much of the jam.
It will look a bit patchy but that is fine.
Bake for 35 minutes until pale golden and leave to cool in the tin.
Once cool, sift the icing sugar into a bowl and stir in 100ml of water with a little food colouring to a thick but runny icing and when you have achieved the preferred colour pour everything over the traybake and tilt the tin to evenly spread.
Dot over a little more food colouring, then use a skewer or toothpick to swirl and marble the top.
Leave to set overnight until the icing is hard then cut into bars.