In the bowl of an electric mixer, beat butter and powdered sugar until well combined, scraping the bowl as necessary.

Beat in the vanilla bean, salt, and lemon zest.

Place a fine mesh strainer over the mixing bowl. Add the hard boiled egg yolks and use a spoon to push the yolk  through the strainer, creating a fine dusting of yolks. Beat the yolks into the butter mixture on low speed.

Add cornstarch and flour to the bowl. Beat on low speed until a crumbly dough begins to form. The dough does not need to be cohesive, but all the powdery ingredients should be incorporated.

Turn the dough out onto a lightly floured countertop or pastry mat and gently knead until the dough forms a ball. Press the ball down with the palm of your hand into a thick disk, then wrap with plastic wrap and refrigerate for at least one hour.

Lightly flour a surface, then roll the dough to about 1/2 inch thick. Cut with flower cookie cutters or whatever shape you desire. Place on a parchment lined baking sheet and bake at 325 degrees for 14-17 minutes, or until firm, but not browned.