Prepare baking sheets with parchment paper. Heat over to 350 Deg F convection ( 375 non-convection). Prepare a piping bag with a large open star tip ( Wilton 1A)

In a bowl of a stand mixer or a large bowl using a hand mixer, cream together the butter, powdered sugar and vanilla sugar until very pale and fluffy. This takes about 8 minutes on medium high speed. Scrape down sides of bowl as needed. 

Add in the egg yolks one at a time and mix well between each addition keeping the mixture light and fluffy. Add in the lemon zest and vanilla and mix well again

Mix together the flour, cornstarch and salt then add this to the butter mixture and mix until well combined.

Transfer some of the dough to the piping bags and hold the top about an inch above the prepared baking sheets. Pipe out about 3 inches of dough making a crescent shape as you pipe. Fill the baking sheet with piped crescents an inch apart. Try to get them as equally sized as you can. 

Bake for 8 minutes. They will still be pale in color but they will be baked.

Cool on baking sheets for a few minutes then transfer to a wire cooling rack. Once they are all baked and cooled heat up the jam so it is very easy to spread

On the smooth side of one crescent spoon about a half a teaspoon or so of the jam and sandwich together with a matching crescent cookie. Repeat for all the cookies. Set in the freezer to firm up the jam. About 20-30 minutes.

Once they are set melt the chocolate in a double boiler on your stove. Bring a pan of water to a simmer then place a glass or non reactive metal bowl over the water but not touching. Add in the chopped chocolate and stir with a spatula until smooth. Once it is all melted turn off the heat and add in the butter. Stir until the butter is melted.  Dip the ends of the sandwich cookies in the melted chocolate, let the excess drip off then place back onto the baking sheet. Dip all the cookies then place back in the freezer to set up the chocolate.  These keep best in the freezer so the chocolate doesn’t make a melty mess at room temperature.  Thaw slightly before serving.