Cream butter & sugar in the bowl of a stand mixer on high speed until light in colour & fluffy.
Add the egg and vanilla and beat until well combined. Add flour, baking powder & salt and mix in well.
Split dough in half and flatten into discs. Place each piece of dough onto a lightly floured baking tray, lined with baking paper or a silicone liner and roll out so that it is around 4mm thick. Place the rolled dough in the fridge to chill for 30 minutes.
Pre-heat the oven to 180°C.
Remove the cookie dough from the fridge and using a 58mm (2 & ¼ inch) or similar round cookie cutter (with crinkled edges if you have one) cut out as many cookies as you can and lay out on a new lined baking tray or peel the dough away from the cut out cookies and spread them out so that there is 2 or 3cm (about 1") between the cookies. Re-roll the leftover dough to make more cookies. You should get around 24 cookies.
Using plunger cutters or tiny cookie cutters (or anything round you can find e.g. an icing tip) cut hearts, flowers, stars or circles out of the middle of half the cookies. These will be the tops.
Bake for 8-10 minutes if baking 1 tray of cookies, or 10-12 minutes if baking 2 trays at once, until the cookies are just starting to go golden on top. You may need to swap the trays over halfway through baking if using 2 trays.
Remove from the oven and allow to cool for 10 minutes before transferring to a cooling rack.
Add the strawberries and sugar to a small saucepan and bring to the boil, stirring occasionally. This should take around 10 minutes, depending on if your strawberries were fresh or frozen.
Stir until the sugar has dissolved and allow the mixture to bubble away, stirring occasionally for around 15 minutes. When the jam starts to thicken up (it will start to bubble like a mud pool!) take it off the heat and allow to cool. Keep a close eye on it after 10 minutes as it can burn on the bottom of the pan. If it starts to go brown, take it off the heat immediately.
Once the cookies and the jam are both cool enough, spoon 1-2 teaspoons of jam on the bottom biscuit (on the side the was on the cookie sheet) and place the biscuit with the shape cut out on top. Press them together so that the jam comes slightly through the shape on top.
Store in an airtight container at room temperature for 1 week. The biscuits will soften slightly each day, which actually makes them taste more like the real thing!