Preheat the oven:
Preheat your oven to 350°F (175°C). Line an 8×8 baking pan with parchment paper.

Make the crumb mixture:
In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the cold cubed butter and cut it into the dry mixture using your fingers, a pastry cutter, or a fork until it resembles coarse sand with pea-sized crumbs.

Press the base:
Pour a little more than half of the crumb mixture into the pan. Gently press it down into an even layer—firm enough to hold together but not packed too tightly.

Add the jam:
Spoon the jam over the base and spread gently. Make sure the jam layer reaches all corners for even flavor.

Add the crumble topping:
Sprinkle the remaining crumb mixture on top. Leave some bigger chunks for a bakery-style crumb.

Bake:
Bake for 30–35 minutes, or until the top is lightly golden and the jam is bubbling at the edges.

Cool & slice:
Let the bars cool completely in the pan before lifting them out and cutting into squares.