Preheat your oven to 350°F and line a baking sheet with parchment paper.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract, mixing until fully incorporated.

In a separate bowl, whisk together the flour, baking powder, baking soda, cream of tartar, and salt.

Gradually add the dry ingredients to the wet ingredients, stirring until a soft dough forms.

Scoop the dough into 12 evenly sized balls and place them on the prepared baking sheet, spacing them about 2 inches apart.

Bake for 12-15 minutes, or until the edges are set and slightly golden.

Let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely while you make your buttercream filling.

Add all frosting ingredients into a large mixing bowl, and mix with an electric hand mixer until light and fluffy. If your frosting seems too loose, add in a little more powdered sugar. If it seems too stiff, add in more heavy cream.

Transfer your frosting into a piping bag fitted with the piping tip of your choice. (If you do not have a piping tip, you can just spread the frosting onto the cookie base with a butter knife.)

Pipe or spread a generous amount of buttercream onto the flat side of one cookie.

Gently press a second cookie on top, flat side down, to create a sandwich. Press lightly to ensure the filling reaches the edges without spilling out.

Repeat with the remaining cookies and filling.

For the best texture, let the cookie sandwiches sit for about 30 minutes to allow the buttercream to slightly set.

Serve and enjoy!