Rub the butter and flour until it looks like sand. Add the sugar and mix until combined. Add the vanilla and egg and lightly knead until combined and without overworking it. The resulting dough should be crumbly.
Heat oven at 180 degrees and, optionally, line a tray with baking paper.
Dump about 2/3 of the dough crumbs in the tray and press with the fingers to an even layer. Spread jam on top. Crumble the remaining dough on top of the jam. Optionally, press it down into the jam to somewhat even it out.
Bake until done.