Cream together the butter, sugar, salt and vanilla seeds in a medium bowl with an electric mixer on medium-high speed for 1 to 2 minutes. (Don't overmix or your cookies will not stay in shape when they bake.) Add the egg and mix just to combine, then add the flour, ginger and cinnamon and mix until it all looks uniform.
Squidge the mixture into a ball and then pat it down into a disc. Wrap the dough in plastic wrap and chill until firm, in the fridge for about 20 minutes or the freezer for 15 minutes.
Preheat the oven to 350 degrees F. Line 3 baking sheets with parchment paper.
Once firm, roll the dough out on a lightly floured surface to about 1/4 inch thick, then cut out your cookies using any cutter you like. You may need to re-roll the dough a couple of times to cut out all your cookies. If the dough gets too soft, put it back in the freezer/fridge to firm up. Put the cut-out cookies in the fridge for 10 minutes to firm up before baking, so they don't spread too much in the oven.
Bake until the cookies are just turning golden brown, 10 to 12 minutes. Let cool completely on a wire rack before decorating with Royal Icing and your favorite sprinkles.
Add the egg whites to a bowl and whisk until frothy soft peaks form. Add the confectioners' sugar gradually, mixing until the royal icing is uniform and nice and shiny. Add the lemon juice and whisk a little more to incorporate.
You can always stiffen up your icing for piping decorations by adding a bit more confectioners' sugar, or loosen stiff icing with a bit of water if needed for spreading or filling areas. To store when not using, put in an airtight container with the plastic wrap touching the surface so a skin doesn't form.