Snip thyme tips from bunch and reserve for decoration, separate leaves from stems and reserve for dough.
In a medium bowl, whisk together flour, baking powder, salt and 1 TBSP chopped thyme leaves and set aside. Zest lemons into bowl of stand mixer, add butter and shortening, and mix to combine. Add sugars and beat until light and fluffy, about two minutes. Add egg yolk and lemon juice and mix until incorporated. Slowly add flour mixture a scoop at a time, scraping down sides and bottom of bowl as necessary until dough is thoroughly mixed.
ROLLING THE DOUGH:
Spray two pieces of parchment paper with cooking spray. Place half the dough in center of one sheet and flatten slightly. Place second sheet on top, then roll to ¼" thick (do not make less than ¼" or the cookies will be too fragile and overcook). Dough will be very sticky. Lay dough with parchment on a sheet pan and place in the refrigerator for 1 to 2 hours or in the freezer for about 15 to 25 minutes, until quite stiff. Repeat with other half of dough. You can make dough ahead and store in refrigerator on sheet pans wrapped in plastic so dough does not dry out (will hold 4 or 5 days).
Once dough is stiff, cut out cookies with larger cookie cutter, then using smaller cookie cutter, cut out center of each cookie to create a wreath. Work quickly to keep dough cold. Have a sheet pan covered with parchment ready for cut out cookies. Remove dough from edges and while cold, pick up cookies with spatula, remove centers, and place on prepared sheet pan. If cookie dough starts to warm up and becomes hard to work with, put back in freezer and work on the next sheet of dough. Keep scraps, push them back together into a ball and re-roll. Dough can be re-rolled several times and chilled before cutting.
Pre-heat oven to 325° with rack in the center of oven. Lightly brush cookies a few at a time with reserved egg white mixed with 1 TBSP water. Sprinkle cookies with sugar crystals (if using), then place thyme tips like a clock at 12, 4 and 8 o’clock. You can start cooking this pan while you decorate the next one. Repeat until all cookies are decorated for each sheet pan. Bake cookies in the center of the oven one sheet pan at a time for 12 to 16 minutes, until edges just start to show color; cookies should be very pale. Add sugar pearls or other decorations while hot out of the oven and press into cookie very gently (some varieties will melt if you add before baking leaving an unsightly crevice). Cool completely before moving to a wire rack.
Once cool, decorate with Royal Icing, which can be made with Pamela’s Vanilla Frosting Mix, to make the dots or other decorations. Let dry totally before storing in an airtight container layered with wax paper.
Chef's Note: To make a more savory version of this cookie add ¼ tsp extra salt and a pinch of cayenne with dry ingredients, and omit the sugar crystals on top. If making with all shortening, cookies will be more fragile and they will rise and fall, leaving a more crackly top. This is not quite as pretty, but they still taste good. This dough must be kept cold to work with, except for when rolling.
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