For the Dough:
In a large mixing bowl, beat the softened butter and sugar for 3-5 minutes until pale, light, and fluffy.
Sift the flour into the butter mixture and gently mix until a soft dough forms.
Divide the dough in half and form each half into a disk. Place each disk between two large sheets of parchment paper or wax paper.
Use a rolling pin to roll each piece of dough to about 3/16-inch (5mm) thickness.
Place the rolled dough on a baking sheet or cutting board and transfer it to the refrigerator to chill for 30 minutes.
Cutting and Baking:
Preheat your oven to 320°F (160°C).
Remove the chilled dough from the refrigerator and peel away the top layer of parchment paper. Lightly dust with flour.
Use a 2-inch round cookie cutter to cut out cookies for the bottom half. For the top half, use a smaller round cookie cutter or a piping nozzle to cut out a window in the center of half of the cookies.
Place the cut-out shapes on a baking sheet and bake for 12-15 minutes, or until golden brown.
Allow the cookies to cool completely before assembling.
Assembling the Cookies:
Fill a piping bag with the buttercream. Pipe the buttercream around the outer edge of the bottom half of the cookies.
Gently place the top part of the cookie (the one with the cut-out center) over the buttercream, pressing lightly to sandwich the two halves together.
Fill another piping bag with the jam. Pipe the jam into the center of each cookie.
Lightly sprinkle powdered sugar over the assembled cookies for a finishing touch.
Optional: Reducing the Jam
To thicken the jam, heat it in a small pot over medium heat for 1 minute until it becomes more liquid.
Whisk in ½ tablespoon cornflour and continue to heat for about 4 minutes, whisking constantly until thickened.
Allow the jam to cool before piping it onto the cookies, as warm jam can melt the buttercream.
For the Buttercream:
In a large bowl, beat the softened butter for 2 minutes until fluffy.
Gradually add the powdered sugar and continue beating until the mixture is smooth and creamy.