Preheat the oven to 350F and grease an 8x8 inch square pan.
Make the base first: Place butter, flour, cornstarch, sugar and salt into a large bowl and beat for about 3 minutes, or until a soft dough forms. To start, the mixture will be crumbly. Keep beating and it will come together into a ball.
Spread the dough into your prepared pan and press firmly into an even layer.
Bake for 10-12 minutes until slightly risen and puffy. Edges will be slightly coloured.
Make the topping: Place oats, brown sugar, flour & butter into a bowl and rub between your fingers until you have a coarse bread crumb texture. Some smaller and bigger lumps are fine.
For the filling: Place jam in a small heat proof bowl and microwave for 20 seconds. (This just makes it a bit easier to spread). Spoon the jam over the shortbread base. Don’t press too hard at this stage as the shortbread is a little soft!
Place fresh cherry halves over the jam. They won’t completely cover the jam, this is fine.
Sprinkle the topping over the jam and cherries. You may not need all of it, it depends whether you want to completely cover the jam or not.
Bake for 20-25 minutes, until the top is golden and the jam is bubbling around the edges. Top should be firm to the touch.
Leave bars to cool in the pan completely, before transferring to a cutting board and slicing into 9-12 bars.
* Cherry Streusel Bars will keep in an airtight container, at room temperature for 3 days, or in the fridge for 5 days.