In a medium bowl, sift the flour and salt.
Add in the cold butter and shortening, incorporating by hand until pea-sized balls form.
Add 8 Tbsp. of cold water. Gently mix together the flour and water until dough comes together. Add more water only if necessary.
Form dough into a flattened disc and cover tightly with plastic wrap. Refrigerate pastry for at least 45 minutes.
Preheat oven to 375°F/190°C.
Roll pastry to 1/4-in. thick. Cut out 10 - 12 circles with a cookie cutter, approximately 3 1/2-in. diameter to fit a standard sized muffin cup.
Gently press cut dough circles into a nonstick muffin pan. Place muffin pan into a freezer for 15 minutes.
Remove pan from the freezer and line each cup with parchment paper (place over the dough). Place pie weights or dry beans to weigh the pastry down and prevent bubbling.
Bake in a preheated oven for 20 minutes; this process is called blind baking.
Remove the parchment and weights from shells and bake until golden, approximately 10 minutes. Remove pan from oven and set aside to cool.
Meanwhile, lightly beat the egg in another medium bowl. Add the sugar, maple syrup, butter, vanilla and salt. Mix well and set aside.
Place crumble topping ingredients into a bowl and rub together until a crumbly texture forms.
Pour maple filling into the pastry shells until 2/3 full. Sprinkle with crumble mixture. Bake tarts until set, about 25 minutes.
Remove from oven and cool 5 minutes in the pan. Remove tarts from muffin pan and place on a wire rack to continue cooling.