Make the Lemon Curd and let cool.

In a large bowl, beat the butter and sugar together with an electric mixer on medium high speed until light and fluffy.

Add the egg, baking powder, salt and vanilla and beat until well combined. Add the flour and mix until combined. The dough will look crumbly. Use your hands to press the dough together.

Divide the dough in half and place each half between two sheets of wax or parchment paper. Roll out to around 1/8-1/4- inch thick, and keeping the paper in place, transfer the dough to a baking sheet. Refrigerate for around 30 minutes.

Preheat the oven to 350 F and position the rack in the middle of the oven. Line two baking sheets with parchment paper.

Working with one portion of the dough at a time, carefully peel away the top layer of wax or parchment paper and cut the cookies into round shapes using 2 to 3-inch cookie cutters. Cut a small circle with a small cookie cutter out of the centre of half of the cookies if desired (to form the 'window). Transfer the cookies to the prepared baking sheets with a metal spatula and bake for around 6 to 9 minutes, one sheet at a time, until light brown around the edges, rotating the pan half-way through. Repeat with the remaining dough, re-rolling the scraps.

Cool the cookies on a wire rack and sprinkle half of the cookies with icing sugar if desired (the ones with the cut outs, if applicable). Spread the underside of half of the cookies with a teaspoon of lemon curd spread to the edges and top with a second cookie to create a sandwich. Repeat with the remaining cookies. Store in a sealed container in the fridge for up to 1 week or in the freezer for up to 1 month.