Line your baking sheets with parchment paper and preheat your oven to 350°F. While the oven heats, measure out all ingredients for both the cookie dough and buttercream frosting, ensuring your butter and egg are at room temperature—this is crucial for proper emulsification and a smooth dough. I prefer using Kerrygold butter for the frosting as it has a richer, more golden flavor that really elevates these sandwich cookies.

In one bowl, whisk together the flour, 1 teaspoon ground cinnamon, ginger, and 1/4 teaspoon salt; set aside. In a separate bowl, beat the 1 cup room-temperature butter with the 3/4 cup sugar for 1-2 minutes until light and fluffy—this creaming process incorporates air and creates tender cookies. Add the egg and vanilla extract, mixing until fully combined and the mixture looks smooth and pale.

Add the dry ingredient mixture from Step 2 to the butter mixture in two additions, mixing gently after each addition until just combined—overmixing develops gluten and toughens the cookies. Transfer the dough to your cookie press and pipe decorative shapes onto the prepared baking sheets, spacing them about 1 inch apart. Chill the piped cookies in the refrigerator for 30 minutes to firm up the dough, which prevents them from spreading too much in the oven.

Bake the chilled cookies for 12-14 minutes until the edges are lightly golden but the centers still look slightly underbaked—they'll continue cooking on the hot pan. Remove from the oven and let the cookies sit on the baking sheet for 5 minutes, which allows them to firm up slightly without becoming hard. Transfer to a wire rack and cool completely for at least 30 minutes before assembling.

While the cookies cool, make the buttercream by beating the 1/2 cup room-temperature butter with the sifted powdered sugar, 2 teaspoons ground cinnamon (I always use freshly ground for a brighter, more complex flavor), milk, and 1/8 teaspoon salt until smooth and spreadable. If the frosting is too thick, add milk a few drops at a time; if too thin, add more powdered sugar. Transfer to a piping bag fitted with a small round or star tip.

Turn half of the cooled cookies upside down on your work surface, then pipe a small mound of the cinnamon buttercream from Step 5 onto the flat side of each. Press another cookie on top, flat-side down, to create a sandwich. The cookies are delicate, so work gently and allow the frosting to set for a few minutes before serving for the best structure.