These cookie tarts are made more decadent by maple caramel sauce.
Preheat oven to 300 degrees. Add butter and sugar to the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until just combined, about 15 seconds. Add flour and salt; mix until dough just comes together, 3 to 5 minutes.
Roll out dough 1/8 inch thick on a lightly floured work surface. Cut out sixteen 3-inch rounds with a cookie cutter. Fit rounds into 2 1/2-inch tartlet pans with 1/2-inch high sides; patch if dough tears, and trim edges. Transfer to a baking sheet; chill at least 30 minutes.
Bake pastry shells, pressing down bottoms occasionally to keep them flat, until light golden brown, 25 to 30 minutes. Transfer to wire racks, and let cool completely. Mound 1/4 cup blueberries in each shell. Just before serving, pour 1 tablespoon caramel sauce over berries.