Heat the raspberries in a medium saucepan, simmering for 10mins (or 15mins if frozen), breaking down the raspberries as you stir continuously.
Press the mix through a mesh sieve into a heatproof bowl, mashing through as much of the mixture as you can before discarding the solids. Allow this to cool on the side while you make the shortbread base.
Pre-heat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
Mix shortbread base ingredients together and press into a lined 8inch square tin. Make sure to press all the way to the edges. Use a fork to prod little holes in the base to help cook.
Bake the base for 20mins until lightly golden. While the base cooks, prepare the filling.
In a large mixing bowl, beat together the eggs, sugar and flour until fully combined.
Add the lemon juice and cooled raspberry liquid into the egg mix, beating together until smooth.
If using, add in a few drops of red food colouring to the filling mix and stir until combined. Add a few more drops at a time until the desired colour is reached.
Once the base has cooked, pour the raspberry mix over the top and return to the oven to cook for a further 25-30mins until the bars have set and there is no wobble in the middle.