For the cookies:

1. Preheat and Prep: Preheat the oven to 350°F (177°C) and prepare a large cookie sheet with parchment paper.
2. Make the Cookie Dough: In the bowl of a stand mixer fitted with a paddle attachment, whip up the cake mix, eggs, and oil until just combined.
3. Shape and Bake: Spread the sanding sugar out on a plate. Scoop up the cookie dough using a 1-inch cookie scoop and roll into a ball. Roll the cookie dough ball into the sanding sugar and place onto the prepared cookie sheet (about 3 inches apart). Bake for 7-9 minutes, then let the cookies cool on a wire rack completely.

For the frosting:

1. Make the Buttercream: While the cookies bake and cool, make the buttercream frosting. In a large bowl, cream together the butter and salt until light and fluffy. Gradually add the powdered sugar, 1 cup at a time, mixing on low speed until combined. Add the vanilla extract, almond extract, and heavy cream. Beat on high speed until smooth and creamy.

Assemble the Cookies:

1. Pipe the Frosting: Once the cookies are completely cool, match them up in pairs of similar sizes. Fill a piping bag with the almond buttercream frosting and pipe a generous amount onto the flat side of one cookie in each pair.
2. Sandwich and Serve: Gently top with the matching cookie to create a sandwich. Serve immediately or store in an airtight container at room temperature for up to 3 days.