Firstly, place the butter and both sugars together into a large bowl and whisk together using an electric hand-whisk until well combined and fluffy. Then whisk in the egg and vanilla for 1-2 minutes until the mixture is paler in colour. Then add the flour, baking powder, bicarbonate of soda and salt and mix again until well combined. Finally, stir the chocolate chips in by hand.
Cut strips of baking paper and place into the holes of a deep 6-hole muffin tin. (This will help you get the cookie pies out once they are baked.) Then divide the cookie dough into 6 balls - they should be about 140g each. Then using 90g of the dough from each ball, press into the muffin tin hole, using your hands to press the dough into the base and up the sides. You should end up with 6 cookie dough 'cups' in the muffin tin holes.
Divide the Biscoff spread into 6, and evenly spoon into each cookie dough 'cup'. (This is around 40g each of spread.) Then gently press down/over the exposed cookie dough sides of each 'cup', so it meets the level of the Biscoff spread. Finally, with the remaining 50g from each cookie ball, flatten into a disc shape between your palms and place over each cookie dough 'cup'. Press fairly firmly down around the edges to seal and gently press over the Biscoff filling. Cover the whole tin in a sheet of clingfilm, and then place into your freezer to chill for 1 hour. (At this point the cookie pies can be left in the freezer for up to 1 week.)
While the cookie pies are chilling in the freezer, preheat your oven to 200C/180C Fan/Gas Mark 6. Then bake for 25 minutes until golden and slightly risen. Remove from the oven and place the muffin tin onto a cooling rack. Sprinkle the cookie pies with sea salt flakes, and then leave to cool completely in the tins.
Once the cookie pies have cooled to room temperature, carefully remove them from the tin using a small palette knife to loosen the edges if needed, and pulling on the baking paper strips.