Combine the flour, icing sugar, and salt in a large mixing bowl. Add the cold butter cubes and rub the mixture between your fingertips until it resembles coarse breadcrumbs with no large butter chunks remaining.

Add the egg yolks, sour cream, and vanilla extract. Mix with a fork, then bring the dough together with your hands until it forms a smooth ball. Do not overwork it.

Flatten the dough into a disc, wrap in plastic wrap, and refrigerate for at least 2 hours or overnight.

Heat the oven to 180 C / 355 F. Line two baking sheets with parchment paper.

Place the dough between two sheets of parchment and roll out to about 3 mm thickness. Keep the second half of the dough in the fridge while you work with the first.

Cut an even number of rounds using a 6 cm round cutter. From half of the rounds, cut a small window in the center using a 2 cm cutter - these will be the tops.

Transfer all cut cookies to the prepared baking sheets. If the dough has softened, refrigerate the sheets for 15 minutes before baking.

Bake for 10 to 12 minutes until the edges are just pale golden and the surface looks set and dry. Cool completely on a wire rack before filling.

If the jam is very runny, warm it in a small saucepan over low heat and stir for 3 to 4 minutes until thickened. Let it cool slightly before using.

Dust the window-cut top cookies lightly with icing sugar.

Spread about 1 tsp of jam onto each plain bottom cookie, spreading just to the edges. Press a dusted top cookie gently on top, aligning the edges. Let the cookies sit for 20 minutes before serving so the jam firms up slightly.