Preheat the oven to 350˚F or 180˚C and prepare a 9x9 in or 23x23cm pan by greasing it lightly and lining it with baking parchment.
Melt two tablespoons of the butter and set aside to cool.
In a bowl, beat together the rest of the butter and the two sugars until fluffy.
Add in the egg, melted butter, and vanilla extract and beat again.
Sift 2 cups or 250g of the flour, baking powder and salt into the bowl. Mix with a wooden spoon until combined.
Bring the mixture together using your clean hands to make a dough and divide it into two equal pieces.
Use your clean hands to spread out half of the dough in the baking pan. Prick the dough with a fork to prevent bubbles from forming while baking.
Spread the peach jam on top.
Mix in the remaining flour into the dough and use a fork to make a knobbly crumble.
Sprinkle the crumble on top of the jam.
Bake for 20 minutes, until golden brown on top.
Leave to cool completely then lift it out with the parchment paper. Cut into squares.
