Whisk flour, baking soda, salt, and spices together
Cream butter and brown sugar just until smooth
Mix in molasses, egg, and vanilla
Add dry ingredients and mix just until combined
Divide dough, wrap, and chill 1½–2 hours
Roll dough to ¼-inch thick and cut into gingerbread man shapes
Place cut cookies on a parchment lined baking pan
Chill the cut cookies for about 15-20 minutes in the freezer
Bake at 340°F (170°C) for 10–11 minutes until set and matte
Cool 5 minutes on pan, then transfer to a cooling rack
Whisk powdered sugar and meringue powder together
Add 4½ tbsp water and vanilla; mix 1 minute on low speed, then 2 minutes on medium speed until smooth and glossy
Adjust royal icing to desired consistency
Pipe eyes, mouth, and details onto cooled cookies using royal icing
Use the icing as glue to adhere gumdrop buttons to the cookies
