Sift together the flour, icing sugar and salt. Mix in the lemon zest . Set aside.

Rub the butter into the dry ingredients with your fingers until it is well incorporated and the mixture becomes crumbly, like a coarse meal.

Whisk together the egg yolks, lemon extract and vanilla extract.

Add to the crumbly dry mixture and mix in until a soft dough forms.

Wrap the dough in plastic wrap and chill in the fridge for at least a half hour. Roll the dough out between two pieces of lightly floured parchment paper to about 1/8 of an inch thick.

Cut out cookies using a 3 inch cutter and place on a parchment lined baking sheet.

Cut circles or hearts or whatever shape you like out of the centers of half of the cookies.

Bake at 350 degrees F for 15 - 20 minutes, just until they start to brown slightly at the bottom edges but are still very pale.

Remove from the oven and spoon 1 rounded teaspoon of the blueberry jam onto the middle of the bottom halves of the cookies and spread out only slightly to about 3/4 of an inch from the edge of the cookie. Carefully lay the top halves of the cookies onto the jam and press down lightly.

Return to the oven for an additional 5 to 6 minutes. This will heat up the jam to a point where it will stick the two cookie halves together.

Cool on the baking sheet for 10 minutes before transferring the cookies to a wire rack to cool completely.

Store in airtight containers.