Preheat oven to 200°C/180°C fan forced. Line a large baking tray with greaseproof paper. Dust a clean workbench with the extra Fate reserved for kneading.
Place Fate, custard powder, sugar, and margarine into a large mixing bowl. Using fingers, rub margarine into the dry ingredients; 3-5min; until well combined.
Tip mixture onto the dusted workbench, knead 1-2min; or until a large ball is formed. Using a sharp knife, slice ball in half. Reshape each half into a ball.
Between two sheets of greaseproof paper, roll one half of the dough to a ¾ cm thickness.
Cut 14 biscuits using the cookie cutter. With a small spatula, lift biscuits onto the baking tray (you may need to re-roll the dough which remains after cutting first round to obtain 14 cookies).
Repeat steps 4 and 5, using second half of the dough.
Bake 12-15min; until tops are just golden. Remove from oven and transfer to a wire rack to cool.
Spread 1 tsp jam over the base of a biscuit. Repeat until you have 14 jam topped biscuits (half of the biscuit batch). Top each of these with a second biscuit to complete the jammy biscuits. Sprinkle with icing sugar if using.
Try spreading a little whipped cream over the jam before sandwiching the cookies together. This makes a truly decadent biscuit!
Depending on the occasion, you may like to experiment with a different shape of cookie cutter. There are so many available, you will be able to find something to suit your needs.
These biscuits stay fresh in an airtight container for 3-4 days. If it is warm, spread with jam just before eating.