Preheat the oven to 150°C Fan and line a baking tray with baking paper.
In a mixer, cream together the butter and sugar until well combined.
Add the egg, egg yolk and vanilla extract and whisk again on medium-speed until well combined. Try not to overmix as an over aerated mixture will give too much rise to the cookies.
Blend the freeze-dried strawberries in a blender until a powder forms or, if larger pieces are desired, crumble roughly into smaller pieces.
Add the flour, ground almonds, strawberries, salt and xanthan gum and mix until a soft dough forms. If using a stand mixer, the dough handle can be used.
Roll the dough out to a 5mm thickness between two pieces of baking paper so as to avoid using any extra flour as this will dry out the dough. Place the rolled out dough into the fridge to chill for at least 30 minutes to 1 hour.
Once chilled, cut out the biscuits using fluted cookie cutter. Cut one top and one base per sandwich cookie.
Place on the baking tray at 1 inch intervals. Gather any cut-offs into a ball of dough and place in the fridge to chill during batches.
Bake the cookies on the middle shelf for 8-10 minutes. The cookies will still be pale in colour but starting to turn golden around the edges.
Allow the cookies to cool for several minutes before transferring them to cool completely on a cooling rack before filling.
For the filling, combine all of the ingredients for the buttercream filling in a mixer. It is important that the butter is softened to room temperature for this. Whisk up the buttercream filling until light and fluffy and all ingredients have been well combined.
Transfer the mixture to a piping bag fitted with an open-star nozzle, and pipe the buttercream in a swirl onto the base cookies. Place the top cookies on top and dust with a light coating of icing sugar if desired.