In a large bowl, combine the butter, shortening, flour, and salt. Use a pastry blender or two knives to cut the butter and shortening into the flour until the mixture resembles coarse crumbs.
Add the cold water, one tablespoon at a time, and mix until the dough comes together. Form the dough into a disc and wrap in plastic wrap. Refrigerate for at least 30 minutes.
Preheat the oven to 350°F. Line a baking sheet with parchment paper.
In a medium bowl, combine the ground almonds, sugar, almond extract, and egg white. Mix until well-combined.
On a lightly floured surface, roll out the chilled dough to about 1/8 inch thick. Using a sharp knife, cut the dough into 4-inch-long strips, then cut the strips into triangles.
Place a teaspoon of the almond filling in the center of each triangle. Fold the dough over the filling and pinch the edges to seal. Place the cookies on the prepared baking sheet and brush each cookie with a light coating of egg white.
Sprinkle the tops of the cookies with the sliced almonds and bake for 15-20 minutes, or until lightly golden brown.
Allow the cookies to cool before serving.
Interesting Facts
Dutch Letter Cookies are a traditional Dutch treat, enjoyed during the Christmas season.
The cookies can be filled with different ingredients such as poppy seeds, raisins, or nutella.
The cookies are shaped like the letter S, which is the first letter of the Dutch word for Christmas, "Sinterklaas".
The cookies can also be made with puff pastry dough instead of traditional pastry dough.