In a medium pot, heat the milk, cream and lemon rind on medium/low heat, until very hot but do not boil. Remove from heat and let cool to warm.

In a medium pot add the yolks and sugar, whisk together until combined, then add the flour and vanilla, whisk again until smooth.

Place the pot back on the stove on medium/low heat, and pour the warm milk/cream mixture through a sieve to remove the lemon rind (discard). Whisk continuously until thick.

Remove from heat and let cool for about 15 minutes. Then move to a glass bowl, cover with plastic wrap (make sure the plastic is touching the cream) and refrigerate for approximately 1-2 hours.

While the pastry cream is in the fridge, make the cookie dough. In a mixing bowl or food processor whisk together the flour, baking soda, sugar, salt and zest, then add the yolks and butter. Pulse or mix to form a compact dough.

Move the dough to a flat surface and form into a dough ball. Wrap it in plastic and refrigerate for approximately 45-60 minutes.

Pre-heat oven to 350F (180C). Line a large cookie sheet with parchment paper.

Remove the dough and pastry cream from the fridge. Roll the dough on a lightly floured surface to a ⅛" thickness, make 20 cut outs with a medium round cookie cutter.

Give the pastry cream a stir, then place a heaping teaspoonful of it in the middle of 10 of the dough circles and cover each one with another round.

Seal the edges with wet fingers, place on the prepared cookie sheet. Bake for approximately 12-14 minutes.

Let cool on the cookie sheet, then move to a wire rack, when cool dust with powdered sugar or drizzle with the lemon glaze.

In a small bowl mix together the powdered sugar, cream and lemon juice. Add more powdered sugar or lemon juice depending on desired thickness.