Preheat the oven to 180 C / Gas 4. Line the bottom of a a 23cm (9 in) square dish/brownie tin with greaseproof paper.
Whisk together butter and sugar until creamy. Whisk in the egg, oil and vanilla until fully combined.
Mix in baking powder and 250g flour a little at a time until combined.
Using your hands, combine the mixture and pick up all the mixture from the edges of the bowl. Divide the mixture into just over half.
Place the larger piece into the dish pressing it as flat as possible and covering the entire bottom of the dish. Use a fork to prick the base to prevent bubbling.
Spread the jam over the top thoroughly and consistently.
Add the remaining 100g flour to the remaining mixture and mix together into crumbs with your hands. Sprinkle the crumbs over the top of the jam.
Bake until golden brown, about 15 to 20 minutes. Leave to rest at least 15 to 20 minutes before cutting, as jam is almost like molten lava and sticks to anything.