Cookie Dough and Baking
1-Beat the butter and sugar for about: Beat the butter and sugar for about 3 to 5 minutes until pale, light, and fluffy.
2-Sift the flour into the beaten butter: Sift the flour into the beaten butter mixture and mix gently to combine forming a soft dough.
3-Divide dough in half, form each half: Divide dough in half, form each half into a disk and place each between 2 large sheets of parchment paper or wax paper.
4-Use a rolling pin to roll each: Use a rolling pin to roll each piece of dough to 3/16 inch (5mm) thickness.
5-Place dough sheets on a baking sheet: Place dough sheets on a baking sheet or cutting board and transfer them to the refrigerator to chill for 30 minutes.
6-Peel away the top layer of parchment: Peel away the top layer of parchment paper from your dough. Dust and rub lightly with flour.
7-Use a 2-inch round cookie cutter to: Use a 2-inch round cookie cutter to cut out cookies (bottom part). With a smaller round cookie cutter or piping nozzle, cut a window out of half the shapes (top part).
8-Place the cutout shapes onto a baking: Place the cutout shapes onto a baking sheet and bake for 12 to 15 minutes in a preheated oven of 320°F (160°C) until golden brown.

Buttercream
9-In a bowl beat the butter or: In a bowl beat the butter or shortening for 2 minutes until fluffy and softened.
10-Gradually add the powdered sugar and continue: Gradually add the powdered sugar and continue to beat this mixture until smooth, creamy, and well-combined. (Add vanilla extract if using).

Jam Filling (Optional)
11-Heat the jam in a small pot: Heat the jam in a small pot on medium heat for 1 minute.
12-Add the cornflour/cornstarch directly into the heated: Add the cornflour/cornstarch directly into the heated jam and whisk continuously for 4 minutes until thickened.
13-Allow the heated and reduced jam to: Allow the heated and reduced jam to cool before piping it into the baked cookies.

Assembly
14-Fill a piping bag or disposable plastic: Fill a piping bag or disposable plastic lunch bag with buttercream. Snip the tip of the bag to allow the frosting to flow freely.
15-Pipe the buttercream along the outer edge: Pipe the buttercream along the outer edge of the bottom part of the cookies.
16-Take the top part of the cookie: Take the top part of the cookie (those with the cut-out center) and place it over the piped buttercream. Press lightly and gently to sandwich the cookies.
17-Fill another piping bag or disposable plastic: Fill another piping bag or disposable plastic lunch bag with the jam. Snip the tip. Pipe the jam to fill in the center of the cookie.
18-Sprinkle castor or powdered sugar over the: Sprinkle castor or powdered sugar over the assembled cookies.