Pour the water into a small bowl. Sprinkle the gelatin powder over this and set aside to bloom for 5 minutes.
In a medium pot, add the milk and sugar. Heat over a medium heat whilst stirring until gently steaming. Turn the heat down to low and pour in the gelatin mixture, stir together until the gelatin has melted inches.
Remove from the heat and pour in the heavy cream.
Strain into a bowl resting in an ice bath. Let sit for 1-2 hours cool, stirring occasionally, until thick but still pourable. (If it isn’t thickening, chill in the fridge for 30-60 minutes to speed it up).
While the panna cotta filling chills, make the cookie cups:
Preheat the oven to 350 degrees F. Spray a non-stick 24-cup mini muffin pan with oil or cooking spray.
In a large bowl, cream the softened butter and sugar together until smooth. Add the egg, vanilla, salt and baking powder, mix to combine.
Add the flour and stir to get a thick dough. Fold in the chocolate chips until evenly distributed throughout the dough.
Divide the dough between the wells in the muffin tin – you should be using about 1 level tablespoon of dough for each well.
Use your fingertips to press the cookie dough in each well into a cup shape.
Place in the freezer for 10-15 minutes.
Bake for 10 minutes – the cookie dough should still be a bit soft in the centre but the edges should be starting to set. They may rise and lose some of their ‘cup’ shape, this is okay!
While the cookies are still soft, use the end of a wooden spoon (or the end of a pestle from a pestle & mortar) to push a large indent back into the centre of each cup.
Return to the oven for an additional 3 minutes.
Let cool in the pan for a few minutes then remove from the pan using a small spatula or butter knife. Place into a wire rack to cool completely.
Fill the cups:
Transfer the thickened, pourable panna cotta filling to a jug.
Fill the cooled cookie cups with the panna cotta filling up to the brim.
Place onto a tray or large plate and transfer to the fridge to chill for at least an hour until the panna cotta has completely set.
Store in the fridge.
Wilton 24 cup mini muffin pan.
Buy it on Amazon: https://www.amazon.com/Wilton-Perfect-Results-Non-Stick-Cupcake/dp/B004EBS16G.