In a large bowl, cream the butter and sugar until well blended and light. Blend in the almonds, almond extract, and egg. Stir in the flour to form a dough.
Turn the dough out onto a lightly floured surface and knead briefly until it forms a smooth ball. Add a bit more flour if needed to make a stiff dough. Gather the dough into a ball, wrap, and chill for 30 minutes.
Lightly butter or coat with nonstick spray 2 dozen fluted sandbakkel tins or tart tins. Pinch off parts of the dough and, using your thumb, press into the tart pans to make a thick, even layer.
Place filled tins on a baking sheet for easier handling. Bake at 190C for 12-15 minutes or until the sandbakkelser are golden. Remove from oven and allow them to cool in the tins. To remove the tins, gently tap the tin until the tart comes out.
Serve unfilled (upside down on a serving plate to reveal the impression from the pan), or invert the tart shells so that the cavity is upright. Just before serving, spoon the chocolate mascarpone mousse into the tart shells.
This recipe makes 24 sandbakkelser.