Place the Tenderflake tart shells out of the freezer and place them on two baking sheets lined with parchment paper.
Preheat the oven to 375°F (190°C).
In a separate mixing bowl, whisk together the brown sugar, maple syrup, melted butter, eggs, vanilla extract, vinegar, until smooth. Stir in any optional ingredients, if desired. I love to use golden raisins.
Spoon the filling mixture into each pastry shell, filling each one about 2/3 full.
Bake for 25-30 minutes, or until the pastry is golden and the filling is set.
Allow the tarts to cool in the muffin tin for a few minutes, then transfer them to a wire rack to cool completely.
Serve the butter tarts at room temperature, and enjoy!
The tarts also freeze well. Store them in a Ziploc bag for space saving. They also taste great when eating them frozen!