Whisk together flours, starch, xanthan gum, baking powder, and salt until well combined, set aside.

In the bowl of a stand mixer fitted with a paddle attachment, cream together butter and sugar on medium speed until smooth and fluffy. Note: You can also use a large bowl and an electric mixer, if you don’t have a stand mixer

Add in egg, beat well. Add vanilla extract, and mix until well incorporated and smooth.

Slowly add dry ingredients mix to the mixing bowl of wet ingredients, and carefully mix until well incorporated and smooth.

Wrap the cookie dough in plastic wrap, chill for 1 hour.

Preheat oven to 350 F (180 C), line baking sheets with parchment paper.

Generously sprinkle clean work surface with corn starch, roll dough to ⅛" thick.

Use cookie cutters to cut out rounds, place cookies 1" apart on prepared baking sheets.

Gather remaining dough into a smooth ball (don’t overwork it!), re-roll. Repeat until all the dough is used up.

Bake cookies for 8-10 minutes, or until bottoms look lightly golden.

Allow cookies to cool on the cookie sheet for at least 5 minutes before moving. You can place the pans on a wire rack if you like - a cooling rack allows air to flow under the pan.

Cookies need to cool completely to room temperature before filling

In a medium bowl, whip butter until smooth. Add vanilla extract and salt, and mix until incorporated.

With the mixer on low speed, slowly add powdered sugar a little bit at a time, until incorporated completely. Beat on high for 1 minute – mixture will be very, very thick.

Lower mixer speed to lowest setting, and slowly add water. Once incorporated, check for consistency. Add more water or powdered sugar to achieve the consistency you want.

Spoon prepared filling into a pastry bag. Cut the tip off and pipe a small amount of filling onto the bottom of one cookie. Top filling with another cookie.

Repeat till all of the cookies are sandwiched.