Make the Dough:

In a large bowl. Beat the softened butter and sugar together. For about 3-5 minutes. Until the mixture. Turns pale, light, and fluffy.

Sift the flour. Into the butter-sugar mixture. And gently fold to create. A soft, smooth dough.

Rolling Out the Dough:

Divide the dough. Into two equal parts. And form each into a disk. Place each disk. Between two large sheets of parchment. Or wax paper.

Using a rolling pin. Roll each piece of dough. To about 3/16 inch (5mm) thick.

Chill the Dough:

Transfer the rolled dough. Still between the sheets of parchment. Onto a baking sheet. Or cutting board. And refrigerate for 30 minutes. To firm up.

Cut the Dough:

Once chilled. Remove the dough. From the refrigerator. And peel away. The top layer of parchment paper. Lightly dust the dough. With flour.

Use a 2-inch round cookie cutter. To cut out the bottom layer of cookies. For the top layer. Use a smaller round cookie cutter. Or piping nozzle. To create a window. In the center of half the cookies.

Bake:

Place the cut-out cookie shapes. On a baking sheet. And bake in a preheated oven. At 320°F (160°C). For 12-15 minutes. Or until they are golden brown.

Assemble the Cookies:

Fill a piping bag (or a disposable plastic bag. With a snipped tip). With the buttercream frosting. Carefully pipe the frosting. Around the edges. Of the bottom cookie layers.

Place the top cookie layers (with the cut-out windows). Over the frosted bottoms. Gently pressing. To form a sandwich.

Fill another piping bag with jam. And pipe it. Into the center of each cookie. Sprinkle powdered sugar. Over the top. For a finishing touch.

Optional: Thicken the Jam

If you prefer. A thicker jam filling. Heat the jam. In a small saucepan. On medium heat. For about 1 minute. Until it becomes liquid.

Mix in 1/2 tablespoon of cornstarch. And whisk continuously for 4 minutes. Until the jam thickens. Allow it to cool. Before piping onto the cookies. To avoid melting the buttercream.

Make the Buttercream:

In a bowl. Beat the butter (or shortening). For 2 minutes. Until fluffy. Gradually add the powdered sugar. And continue beating. Until the mixture becomes smooth, and creamy. And fully combined.