Lemon Cookies:

Line 1 large tray, or 2 smaller trays, with parchment or silicone baking mats.

In a medium bowl, whisk together all purpose flour and custard powder. Set aside.

In a large mixing bowl or the bowl of a stand mixer, whip butter on medium-high speed until light and fluffy. Add the granulated sugar and mix on medium-high speed until creamy. Add the egg yolk and lemon zest and mix until fully incorporated.

Add the dry ingredients and mix on medium speed until large clumps form.

Mix in the lemon juice until fully incorporated, then use your hands to finish bringing the mixture together into a smooth, slightly sticky dough.

Shape dough into a disc. Wrap in clingfilm/plastic wrap and refrigerate at least 1 hour, or overnight.

On a silicone rolling mat, use a rolling pin to roll the chilled dough to about 1/8-inch thickness.

With a 1 3/4-inch fluted round cutter, cut dough into circles. Re-roll scraps and cut remaining circles.

Arrange on prepared trays.

Refrigerate trays for 30 minutes, or freeze for 15 minutes. Meanwhile, preheat oven to 350° F / 176° C.

Bake in preheated oven, one tray at a time, for 11-13 minutes, or until very lightly golden brown around the edges. Do not overbake.

Transfer to a wire rack to cool. Meanwhile, prepare the lemon cream filling.

Lemon Cream Filling:

Pass confectioner’s sugar through a fine mesh sieve to remove lumps and hard bits of sugar. Set aside.

In a microwave-safe bowl, heat lemon juice until very warm to the touch. Add custard powder and stir until mixture turns a vibrant peach color. Set aside.

In a large bowl or the bowl of a stand mixer, whip softened butter on high speed until soft and fluffy. Add confectioner’s sugar, lemon-custard powder mixture, lemon zest, and lemon oil, if using. Mix until creamy and well-combined. Filling will be pale orange or peach in color.

Scoop lemon cream filling into a piping bag fitted with a star or other decorative piping tip.

Divide cookies into pairs. Pipe cream on the flat side of half of each pair. Top with the second cookie. If desired, dust the tops with confectioner’s sugar just prior to serving.

Store in an airtight container at room temperature for up to 5 days, or layer between sheets of parchment and freeze in an airtight container for up to 3 months.