Here’s a recipe for EU opt-in cookies
You can control the level of gooey-ness by altering the amount of condensed milk. You might also need to bake for a bit longer than 20 minutes as the blueberries make it a bit soggy (and I got carried away as I love blueberries).
I wasn’t able to find any blue food colouring that wasn’t nastily artificial, and my attempts with yellow and purple ended up looking like weird alien food from a mid seventies episode of Dr Who. Still tasted good though.