1. In the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar together until fluffy, about 3 minutes, scraping down the sides of the bowl as necessary.
2. Add the egg, followed by the ground almonds, salt and almond extract. Mix until incorporated.
3. Add the flour and mix on low until just combined.
4. Remove the bowl from the mixer and knead the dough gently with your hands inside the bowl for a minute or two until it all comes together into a cohesive whole. Remove the dough from the bowl and place on a large sheet of plastic wrap. Wrap tightly and chill dough for at least 30 minutes.
5. Preheat oven to 375 degrees. Line a rimmed baking sheet with a piece of parchment paper and have 25 or so tart pans ready to go. Remove chilled dough from the refrigerator and tear off a piece that is approximately the size of a walnut. Pat the small piece of dough into the tart pan using your thumbs and fingers. The dough should be uniformly about ⅛th inch thick across the surface of the tart pan. Create a nice even edge at the top of the pan (see video for a visual demonstration).
6. Place 8-10 filled tart pans on a rimmed baking sheet lined with a piece of parchment paper and slide them in the oven for 12-15 minutes or until they are light golden brown in color.
7. Remove the baking sheet from the oven and allow the mandelmusslor to cool before handling. Once they are cool enough to touch, tip them upside down and remove the tart shell from the pan. This may require a little help from your fingers or by tapping and/or wiggling the pan a little to loosen the tart shell from the sides. This may take a little practice. Let the mandelmusslor cool on a wire baking rack. Repeat with remaining tart pans, baking 8-10 on a baking sheet at a time. Fill (or don't fill) as desired.