For the gluten free chocolate chip cookies:

1. Mix butter, brown sugar and white sugar with a mixer at medium speed until creamy, about 3 minutes. Add vanilla and eggs, mixing at low speed until incorporated, about 1 minute.
2. In a bowl, whisk together baking soda, salt, and almond flour. Pour into mixer and mix on low speed until just combined. Stir in chocolate chips.
3. Line two baking trays with parchment paper or baking paper. Use an ice cream scoop to scoop three cookies onto the first tray. Cover with another piece of parchment paper and press the second baking tray down onto it, until you feel the resistance of the chocolate chips. Remove the additional tray and paper. Repeat with the rest of the cookie dough to make a total of six flattened cookies.
4. Bake cookies at 350 F or 175 C for 12 minutes. Let the cookies cool completely while you make the buttercream.

For the buttercream filling:

1. In a mixer, mix 1/2 cup butter and powdered sugar at the lowest speed until smooth. Add vanilla, salt and milk and mix to combine. Optionally, spoon into a piping bag.

To make the chocolate chip cookie cake:

1. Spread or pipe a dot of buttercream onto a plate or cake stand. Press the first cookie onto it to attach it.
2. Cut each of the remaining cookies into 6 equal portions.
3. Pipe or spread buttercream onto the first cookie and press the next pre-cut cookie pieces onto it. Continue alternating buttercream and cookie pieces, lining the pieces up above each other to make the cookie cake easy to cut.
4. Store, covered, at room temperature for up to 2 days or in the fridge for up to a week.