Using a small round cookie cutter or the rim of a small glass (shot glasses will work well), cut small circles out of the puff pastry and place them on lightly greased baking trays. Put them in the fridge while you make a choux pastry.
To make the choux pastry, place the milk, water and butter in a medium saucepan and heat it up to melt the butter, but do not bring it to the boil. Tip all of the flour into the saucepan and beat it quickly with a wooden spoon or spatula until it all comes together. Cook the mixture for about a minute then transfer it to the bowl of a standing mixer to cool down a bit (about 5 minutes). Once it is cooled, add the eggs, one at a time, mixing it well after each addition. You should have a smooth and glossy mixture.
Transfer the pastry into a piping bag with a large, plain nozzle.
Take the puff pastry circles out of the fridge and pipe small balls of the choux pastry on top of them. They should be smaller than the puff pastry circles.
Using the remaining choux pastry, pipe more small, even in size balls (you may have to use separate baking trays for this step, if you can't fit them on the same trays as puff pastry circles. You should have the same amount of pastry circles and choux balls.
Using a wet finger, smooth any peaks on top of the balls.
Place the baking trays in the oven and bake for 15- 20 minutes until puffed up and golden brown.
Take out of the oven, allow them to cool for a couple of minutes, then using a small knife, pierce the bases of the small choux buns, and tops of the buns inside of the puff pastry circles.
For the fillings
Whisk the custard with 300 ml of double cream until it forms stiff peaks. Transfer to a piping bag with a small nozzle. Pipe the filling into both, the small choux buns and the ones inside of the puff pastry.
Place 100 g of Tate & Lyle Caster Sugar in a pan and allow it to melt over high heat until it melts and starts to caramelise. Do not stir it! Working quickly, but very carefully, dip the tops of the small choux buns in the caramel and set aside to set.
Whip the remaining 300 ml of double cream with 2 tbsp of caster sugar and vanilla bean paste. Transfer it to the piping bag with a large star nozzle.
To assemble, pipe some whipped cream on top of the buns in puff pastry, then top with a small choux bun dipped in caramel.
Push a lollipop stick or a skewer through each assembled bite and place in the fridge for about 30 minutes. Enjoy!