In a mixing bowl, combine the egg yolks and sugar and whisk until the sugar is dissolved and the egg yolks become creamy.
Add the yogurt and the gently melted butter and mix well.
In a separate bowl mix the flour, baking powder, and salt. Gradually add the flour mixture to the egg mixture. Knead the dough lightly until it forms a smooth ball that does not stick to your hands. If the dough was crumbly and dry, add a small amount of water.
Place the dough in an airtight container and refrigerate for approximately 1 hour.
In a food processor combine all the filling ingredients and pulse until all ingredients have been blended and you have a walnut paste.
Working on a lightly floured surface, roll the dough out with a rolling pin to approximately 1/8 of an inch. The well-rested dough should be very easy to work with.
Cut the dough into 3-inch circles using a cookie cutter. Fill each circle with about 1 teaspoon of the walnut mixture. Resist the temptation and avoid overfilling the pastry as it will make it difficult to fold. Dampen the edges of the circle with a wet finger to ensure the pockets remain sealed. Fold the circles into halves and seal them shut using a fork or by pinching the edges together using your thumb and index fingers.
Heat the oven to 325°F and carefully place the pastry onto a baking sheet lined with parchment paper and bake for about 15 minutes, or until lightly brown.
Remove the pastries from the oven and allow to cool. Once cooled, using a sifter, gently dust with confectioner’s sugar on both sides and sprinkle with ground pistachios. Arrange on a serving platter and serve.