Cream butter and sugars together with an electric mixer until pale and fluffy. "Build up speed slowly so the mix doesn't fly everywhere, but don't be afraid to use high speed. If you don't go past medium speed, it won't get as light and fluffy." Mix in molasses. Add egg and beat just until combined.
Whisk together dry ingredients in a large bowl. With mixer on lowest speed, add dry ingredients in 3 or 4 batches to butter and sugar. Mix until not quite combined. "You don't want to keep going because then you'll start the gluten development in the flour and get tough dough."
Divide dough into 3 disks. Chill, wrapped in plastic, at least 1 hour.
Meanwhile, make glaze: Put bittersweet chocolate in a large glass bowl with the butter. Melt in a microwave in 2 or 3 bursts of 30 seconds each, stirring after each. Stir in corn syrup. Let cool until it's body temperature and drizzles in a thick ribbon. (If it solidifies, microwave about 10 seconds and stir to liquefy.)
Preheat oven to 350°. On a lightly floured surface, roll each dough disk to a 1/4-in. thickness. Using a snowflake-shaped cutter, cut out cookies firmly, dipping cutters in flour each time to keep them from sticking. "Cut around the outside and work toward the center; you'll get more cookies that way." You can reroll scraps, but for a more tender cookie, blend them gently--don't mash them.
Transfer cookies to baking sheets lined with parchment paper, setting them 1 in. apart, and chill 10 to 15 minutes. "They'll hold their shape better and the edges will be a little sharper." Bake 8 to 12 minutes, rotating once, until just firm to the touch. Cool on racks.
Decorate cookies: Put bittersweet and white chocolates separately into 2 medium metal bowls. Fill 2 medium saucepans with 1 in. water; bring to a simmer. Remove from heat and set bowls on pans; let chocolates melt until about half-melted. Stir each until smooth.
Immediately pour each chocolate into a cornet (a small cone of parchment paper)* or resealable plastic bag with a tiny corner cut off. Decorating one cookie at a time, outline in dark chocolate.
Fill a larger parchment-paper cone with chocolate glaze and use it to fill in cookie. (If you're short on time, you can skip the outlining, pour glaze into a bowl, and swirl the face of each cookie in glaze.)
Decorating one cookie at a time, squiggle white chocolate onto snowflake, then draw the tip of a sharp knife or toothpick through squiggle. Or squeeze out thin curls. "Each one's going to be a little different, but then again, so is every snowflake."
*To make a parchment cornet, roll a triangle of parchment into a cone, starting at a long corner and rolling it over on itself; tape or staple edge. Fold up tip and set in a tall empty glass while you fill it with chocolate. Fold top over, then unfold tip and pipe (snip a larger hole if you need to).
Make ahead: Dough, up to 1 week chilled or 1 month frozen; glazed cookies, up to 3 days, stored airtight in a single layer.
High-altitude note: Add 2 tbsp. flour and take out 2 tbsp. each granulated and brown sugar; this keeps the cookies soft and thick.
Note: Nutritional analysis is per cookie.