In a bowl, cream butter and sugar, then beat in the vanilla essence to taste and the egg yolks. Sift the flour and salt into bowl, blend with a spoon, then work by hand to form a dough. Knead lightly until smooth. Wrap in plastic wrap and chill before filling.
Preheat oven to 180°C/350°F/Gas 4. Grease 2 baking sheets. Roll out the dough on a floured surface to 0.3 cm thick. Cut out 7.5 cm rounds. Put a teaspoon of filling on each round. Brush edges with beaten egg, then bring edges to centre to cover filling and form a three-cornered shape. Place on baking sheets. Glaze with egg. Bake for 20-35 minutes, cool on wire racks.
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