Stir the flour, sugar, and salt together with the paddle attachment of a stand mixer or a pastry blender. Add the butter and mix on low speed until the butter has been worked into the flour mixture. Stop when the butter is the size of peas.

In a small bowl stir together the water and lemon juice. With the mixer on slow speed, add this to the dough all at once. Mix until dough comes together.

Form dough into a bowl and cut in half. Shape into discs and cover with plastic wrap. Chill in the refrigerator for 2 hours to 24 hours.

Preheat oven to 400 degrees F and grease a 12-cup muffin tin. Remove dough from the fridge 20 minutes before rolling.

Roll each disc out onto a lightly floured work surface to ¼ inch thick. Use a 4 ½ inch circle cookie cutter to cut out each circle. Line muffin tins with dough circles and chill the lined tin while preparing the filling.

In a saucepan over medium heat, combine butter and brown sugar. Stir until the mixture is bubbling. Remove from heat.

Whisk eggs, maple syrup, lemon juice, vanilla extract, and salt together in a large bowl. Slowly pour in a bit of the hot mixture and whisk to temper the eggs. Pour in the rest of the hot mixture and whisk until incorporated.

Put a few pecan pieces or raisins in the bottom of each tart shell. Ladle the filling into each shell, about ¾ full.

Bake for 10 minutes at 400 F and then reduce to 375 F and bake for an additional 10-15 minutes, rotating half-way through. Tart filling should be bubbling and crust edges will be browned.

Cool the butter tarts in the muffin tin for 5 minutes. Use a knife to run around the edges of each tart to prevent sticking. Keep tarts in muffin tin until fully cooled.

The butter tarts should be stored at room temperature in an air tight container for up to 3 days.