Using an electric mixer, beat the butter and confectioners' sugar on high speed until fluffy, about 4 minutes. Mix in the cinnamon, baking soda and salt at low speed. Beat in the whole egg, egg yolk and vanilla at medium speed until incorporated. Add the flour and mix at low speed.
Divide the dough in half and shape into 2 disks; wrap in plastic wrap and refrigerate for 20 minutes. Meanwhile, line 2 cookie sheets with parchment paper. Position the oven racks in the upper and lower thirds of the oven and preheat to 325 degrees .
On a lightly floured surface, roll out 1 chilled disk of dough 1/4 inch thick. Using a 5-inch gingerbread girl cookie cutter, cut out the dough. Using a spatula, transfer the cookies to a cookie sheet; refrigerate. Gather and reserve the scraps. Repeat with the second disk of dough. Combine all the scraps, roll and form more cookies.
In a small bowl, beat the egg white until frothy. Brush the cookies with the egg white and sprinkle liberally with the decorating sugar. Bake until light golden, about 15 minutes. Let cool on the pan for 5 minutes, then transfer to a rack to cool completely.