Cut the butter into cubes. Place into a large mixing bowl with both types of sugar.
Whisk until just combined.
Add the egg and whisk until just combined.
Add the flour, corn flour, baking powder, baking soda and salt and whisk until fully combined. You shouldn’t see any little lumps of flour/white dusty bits!
Divide the dough into 8 roughly equal balls. Roll the balls into a smooth, round ball and then flatten with the palm of your hand until they are approx 1/2 inch thick.
Pre-heat your oven to 185oC. Cover a baking tray with tin foil>Baking paper will do the job but tin foil is better if you have some! Place the cookies on the foil and place into the oven for precisely 18 mins!>Cooking time and temperature makes a hugeeeeeeeee difference. Don't play about with it unless you know you've got an oven that is either always too hot or cold! Remove from the oven and leave them alone for at least 15 mins to cool before attempting to touch them!>Under NO circumstances should you try to move the cookies from the tray when you've removed them from the oven!!!! They need at least 10-15mins to cool, during which time they will keep cooking. If you try to move them before, they will crumble and the inside will still be raw. Whilst they are cooling, whisk together the butter, icing, custard powder and vanilla until smooth and paler.>Whisk for at least 5 mins for optimal, creamy taste! Once the cookies are completely cool, pipe/spoon your buttercream onto the underside of half of the biscuits. >Don't put too much buttercream on there, otherwise it makes it hard to actually eat it!!
Add 1 tbsp of jam to the underside of the other half of the cookies. Spread it around a little, but not right to the sides otherwise it will all squish out when you sandwich them together!
Sandwich together 1 of the buttercream cookies and 1 of the jam cookies! Repeat for the rest of the cookies