Ingredients:

For the tart shells:

• 2 cups unbleached flour
• 1/8 teaspoon salt
• 3/4 cup salted butter, chilled
• 4 to 6 tablespoons cold water

For the filling:

• 1 cup rhubarb, chopped
• 1 cup Saskatoon berries or blueberries
• 1/4 cup butter, cut into small cubes
• 1/4 cup brown sugar
• 3 to 4 tablespoons maple syrup
• 1 egg, lightly beaten
• 1 teaspoon vanilla

Directions:

1. To make the homemade tart shells, combine dry ingredients and add butter.  Blend together with a pastry blender or food processor until pea-sized bits of butter are visible throughout.  Gradually add cold water until a dough forms.  Cover dough and chill for 15 minutes or more.  Keeping the other portion chilled, roll out half of dough on a floured surface and cut out 5-inch (12 cm) rounds. [I used a plastic lid and traced it with a butter knife.] Continue to roll out dough until twelve rounds are made.
2. Preheat oven to 370 degrees F (188 C).  Gently arrange circles of tart shell dough into muffin tin cups, fluting the edges.  [I found I preferred the tarts baked without paper muffin liners, but you may line the cups if you wish.]
3. To make the filling, soak chopped rhubarb in boiling water for 5 minutes.  Drain rhubarb and combine with butter cubes in a small mixing bowl.  Add brown sugar and stir until sugar and butter form a paste.  Add berries, maple syrup, egg, and vanilla.  Stir until thoroughly combined.
4. Carefully spoon filling into unbaked tart shells.  Bake at 370 degrees F (188 C) for 22 minutes or until crust is golden.  Allow tarts to cool in the pan for 5 to 10 minutes before removing to a cooling rack.  Serve butter tarts slightly warm or at room temperature.