In a food processor with a steel blade, put flour, powdered sugar, butter, cornstarch, salt and vanilla and process into a crumbly mixture.

Add the yolk and continue to process just until large lumps of dough are formed. If the dough does not combine, gradually add some milk.

Transfer the dough to a lightly floured work surface, combine with your hands and flatten into a disc shape. Wrap with plastic wrap and cool in the refrigerator for about an hour.

Break the chocolate into squares and place in a bowl. Add cream and melt together in a microwave or double boiler until everything is melted and the mixture is smooth.

Add egg and beat well until it is absorbed into the mixture.

Add nuts and mix until evenly distributed.

Chill the filling for two to three hours in the refrigerator or until it reaches the texture for spreading.

On a floured surface, roll the dough into a 1/8-inch- (three- to four-millimeter-) thick leaf. Cut circles six to seven centimeters (about two and a half inches) in diameter and place on a baking pan lined with Gefen Parchment Paper.

Place a teaspoon of filling in the center of each circle, and close to a triangle shape, tightening the edges securely.

Chill the hamantaschen in the freezer for about half an hour before baking.

Heat the oven to 350 degrees Fahrenheit (180 degrees Celsius).

Bake for 15–20 minutes or until the hamantaschen are golden all over.

Cool completely at room temperature. Sprinkle with powdered sugar and serve.