In a large bowl whisk together of flour, baking powder, baking soda and salt; set aside.
910 g flour, 1 tsp baking soda, 1 tsp baking powder, 1 tsp salt
In the bowl of a stand mixer, beat the sugar and butter until fluffy.
440 g unsalted butter, 400 g sugar
Add eggs, one at a time and mix until blended.
5 eggs
Add sour cream, vanilla and lemon juice and blend.
240 g sour cream, 10 ml vanilla or the seeds from one vanilla bean, 5 ml lemon juice
Gradually add flour mixture, alternating with milk, to the mixture in the stand mixer bowl. The dough will be very sticky, somewhere between a thick cake batter and a loose cookie dough – chill for at least 3 hours or overnight.
60 ml milk
Form the dough into about 1 1/2" diameter logs (about 6 11" long logs total). Wrap in waxed paper, twisting the ends to seal. If you have paper towel (or similar sized) tubes, slide each log of dough into its own tube - this helps prevent the dough from developing a flat side while it is chilling. Place all the logs in the freezer for at least 2 hours.
Pre-heat oven to 350°F. Line large baking sheets with parchment paper. Remove one log of dough from the freezer at a time. Using a sharp knife, slice the log into 1/4"-1/3" thick rounds. Arrange the rounds about 2" apart on the baking sheets and bake for about 8-10 minutes. Cookies should be set, but not browned. Transfer to a wire rack and cool completely.
For the glaze:
While the cookies are cooling, lay out a large sheet of parchment paper next to your cook top. Place the assorted decorations in open bowls/containers nearby where they can be quickly accessed.
Place a 3 qt double boiler insert over a pot with 1-2 inches of simmering water in the bottom. Add all of the glaze ingredients, in the order listed, into the double boiler. (Depending on your double boiler, it may be necessary to add the powdered sugar in multiple smaller batches to avoid it flying all over everywhere)
300 ml heavy cream, 100 g shortening, 35 ml lemon juice, 32 ml corn syrup, 20 ml butter vanilla emulsion, 20 ml clear vanilla extract, 5 g popcorn salt, 1820 g powdered sugar
Give the shortening a few minutes to melt, then stir the ingredients together. When the powdered sugar has been mostly incorporated, switch to an immersion blender and mix until the mixture has a satiny sheen and pours off the end of the blender in a continuous ribbon.
Continue to heat the mixture, stirring frequently, until there is no grainy-ness remaining from the powdered sugar and a small amount of the glaze dropped on the edge of the parchment paper forms a set crust within a few seconds.
Place a cookie top side down into the glaze, using a dipping tool or fork, gently flip over - lift out of the glaze and gently shake the excess off. Grab a pinch of decorations and, as you set the dipped cookie on the parchment paper, immediately sprinkle the decorations evenly over the surface of the cookie. You will need to work very quickly before the surface of the glaze sets.
Assorted decorations
Repeat until all the cookies are coated or until all the glaze is used. If necessary, make additional glaze to finish coating all of the cookies.
When the glaze has set enough to cleanly lift from the parchment paper (about 15 minutes), transfer to wire racks to allow air to circulate underneath and the glaze to completely set (about 1-2 hours). Store in an air tight container with waxed or parchment paper between layers.