Make the dough: Place the butter and sugar in a food processor. Blend until combined well and creamed, about 1 minute. Add the egg, vanilla and lemon juice and process until smooth. Mix the flour with the lemon zest in a separate bowl. Add the flour and lemon zest mixture and baking powder into the food processor and blend until the dough comes together, about 5 minutes. Transfer the dough onto a floured counter, roll it into a ball and cover it with plastic wrap. Refrigerate for at least 1 ½ hours. 

Meanwhile, make the filling: Place the dates, water, and sugar into a small pot over medium high heat. Bring the mixture to a boil and reduce heat to medium low. Cook the mixture on a gentle simmer, occasionally stirring, until it thickens to a jam like texture and the liquid evaporates, about 15 minutes. Mix in the vanilla extract and remove the mixture from the heat. Transfer to a heatproof bowl and let it cool in the refrigerator for about 30 minutes. 

Preheat the oven to 375°. 

Shape the cookies: Transfer the dough from the refrigerator. Remove the plastic wrap and place it onto a well floured surface. Roll out the dough to a thickness of ¼ an inch. Using a cookie cutter or the back of a cup, cut out 3 inch circles from the dough. Peel off any excess dough and set aside. Place about 1 teaspoon of the date filling into the center of each circle. Brush the edges of the circle with water. Starting with one piece of dough, roll up 3 edges of the cookie to create a triangular shape and pinch the 3 points of the triangle together with your fingers. Keep the center of the cookie open to reveal the filling. Transfer the cookie to a parchment paper lined baking tray. Continue shaping the cookies with the rest of the dough and place them onto the baking sheet. Use the excess dough to make more cookies by combining the pieces into one piece of dough and repeating the rolling, stuffing and shaping process.

Transfer the cookies to the oven and bake for about 15 minutes until golden brown. Allow the cookies to cool and serve.