Line four baking sheets with parchment paper or silicone baking mats. Set aside.
In a medium mixing bowl, whisk together the flour, cinnamon, ginger, and salt. Set aside.
In a large mixing bowl, beat together the butter and sugar for 1 to 2 minutes, until pale and fluffy. Add the egg and mix to combine. Scrape the sides of the bowl as needed.
Add half the dry ingredients to the wet ingredients and mix until just combined. Add the rest of the dry ingredients and mix until completely incorporated.
Transfer the dough to a cookie press fitted with your design disc of choice. (Not all the dough will fit at once - you'll have to work in batches.). Press cookies onto the prepared baking sheets, about 2-inches apart. I can usually fit 1 dozen cookies per pan.
Put the baking sheets into the refrigerator and chill cookie dough for 30 minutes.
Preheat your oven to 350 degrees F. Bake the cookies for 12 to 15 minutes, or until the edges are still pale and cookies are set. Keep the other baking sheets in the fridge until it's time to bake.
Remove cookies from the oven and let sit for 5 minutes on the pan. Then transfer to a wire rack to cool completely, about 30 minutes. Repeat with remaining cookies.
Once all the cookies are cooled, make the buttercream. Add all the ingredients to a mixing bowl and beat until smooth. Transfer the buttercream to a piping bag or ziploc bag with a corner cut off.
Turn half of the cookies upside down. Then pipe a dollop of frosting onto each upside-down cookie. Place another cookie on top (with the bottom facing the frosting) and press gently to spread the frosting to the edges.
Serve immediately or store in the fridge in an airtight container. I like to chill these cookies so the buttercream sets up and doesn't squish out the sides as much.
You will have leftover buttercream. You can make more spritz cookies and fill them, frost some sugar cookies with it, or add a little on top of your french toast and let it melt! So good!