1. Beat butter and sugar at medium speed with a heavy-duty electric stand mixer until fluffy.
2. Stir together baking soda and 1/4 cup hot water until dissolved; stir in molasses.
3. Stir together flour and next 4 ingredients. Add to butter mixture alternately with molasses mixture, beginning and ending with flour mixture. Shape mixture into a ball; cover and chill 1 hour.
4. Preheat oven to 350°. Roll dough to 1/4-inch thickness on a lightly floured surface. Cut dough with a 1- to 2 1/2-inch snowflake-shaped cutter. Place 2 inches apart on parchment paper-lined baking sheets.
5. Bake at 350° for 13 to 17 minutes. Let cool on baking sheets 2 minutes; transfer to wire racks. Cool completely (about 30 minutes).
6. Spoon Royal Icing into a small zip-top plastic freezer bag. Snip 1 corner of bag to make a small hole; pipe decorations on cookies, or spread cookies with Buttermilk Frosting. Decorate cookies as desired with candies and sugar crystals.