For the cookie dough:

1. In a large bowl, sift together flour, baking powder, and salt. Whisk and set aside.
2. In the bowl of a stand mixer with the paddle attachment, beat butter and sugar together until fluffy. Add egg and egg yolk, lemon zest, lemon juice, and vanilla extract. Beat until combined.
3. Add the dry ingredients, mixing on low speed until a soft dough comes together. Shape the dough into a disk and wrap in clingfilm. Transfer to the refrigerator for at least 30 minutes. In the meantime, prepare your filling.

For the filling:

1. In a small saucepan, combine dates, water, marmalade, and cinnamon. Bring the dates to a boil then reduce to a simmer until the water has absorbed and the dates have softened. Remove from the heat and let cool slightly before transferring to a blender. Pulse until the mixture forms a thick paste. Set aside until ready to use.

Shaping the cookies:

1. Preheat oven to 350 degrees F. Line two baking sheets with parchment paper and set aside.
2. Divide the cookie dough in half, returning the other half to the refrigerator until ready to use. On a lightly floured surface, with a floured rolling pin, roll one-half of the dough out to 1/8-inch thick. Using about a 2-inch round cookie cutter or glass, cut rounds, reserving the trimmings. You should get about a dozen from each half of the dough.
3. Spoon 1 teaspoon of date mixture into the center of each round. To make a triangular pocket, lift the edge of the dough at three points and pinch together, partially covering the filling. Place 1 inch apart on the prepared cookie sheets. Sprinkle the centers of each cookie with Turbinado sugar if desired.
4. Bake the cookies for about 12-14 minutes, until the pastries are lightly browned around the edges. Transfer the cookies to a wire rack to cool before serving.
5. Repeat this process with the remaining dough.