Preheat oven to 180 degrees Celsius (360 degrees Fahrenheit).
In a mixing bowl, combine flour, sugar, and salt. Stir until combined.
Make a well in the center and pour in melted butter. Mix until a soft, crumbly dough forms.
Set aside about a quarter of the dough mixture for the crumble topping.
Press the remaining dough into a lightly greased and lined 8-inch square cake pan, smoothing it evenly to the edges.
Use a fork to make indentations across the surface of the dough.
Spread 1 cup of jam evenly over the dough.
For the crumble topping, use your fingers to rub the reserved dough mixture until small, nubbly bits form.
Scatter the crumble mixture over the jam, leaving some jam visible.
Bake for 20-25 minutes, or until the crumble topping is golden brown.
Let the slice cool in the tin for at least 20 minutes.
Once cooled, dust with icing sugar (optional).
Cut into squares.
