Heat the oven to 200C/180C Fan/Gas 6. Line an 8" square tin.
Make the shortbread by combining all ingredients with a high speed mixer or in a food processor. The resulting mixture should be crumbly. Press into the lined tin in an even layer.
Bake the shortbread for 20 minutes. Remove and let cool on a wire rack.
While the shortbread is baking, make the berry layer by combining all ingredients in a large mixing bowl/the bowl of a stand mixer. It should come together quickly. (Note: if using frozen berries, melt them in the microwave so you can mash them into a liquid/paste).
`Pour this over the baked shortbread layer. Bake for a further 20-25 minutes: until the middle is no longer wobbly. Remove and let cool on a wire rack, then cool in the fridge for at least an hour before slicing.