To make filling: Combine prunes, raisins and water in a small saucepan. Simmer over low heat until the prunes are tender but still firm and liquid has been absorbed, about 10 minutes.Advertisement
Combine the prune mixture, apples, walnuts, sugar, lemon juice and zest in a food processor; process until smooth. Transfer to a small bowl and set aside.
To make cookies: Beat sugar, oil and butter with an electric mixer on medium speed in a medium bowl until smooth. Add egg and beat until smooth. Add vanilla and beat until blended. Sift together flour, baking powder and salt in another medium bowl. Using a wooden spoon, stir the dry ingredients into the sugar mixture until just combined. Gather the dough together into a ball, wrap with plastic wrap and flatten slightly. Refrigerate for 2 to 3 hours or overnight.
Preheat oven to 350 degrees F. Line two baking sheets with baking parchment or coat with nonstick cooking spray.
Divide the dough in half. (Refrigerate the other half.) Roll out on a lightly floured surface to a thickness of 1/8 inch. Cut the dough into circles using a 2 1/2-inch-diameter cookie cutter. Place 1/2 teaspoon of the filling in the center of each circle. Bring the edges together to cover the filling, forming a 3-cornered cookie. Pinch the corners together to seal.
Place the cookies about 1 1/2 inches apart on the prepared baking sheets. Bake until the tops are golden, 10 to 15 minutes. Transfer to racks to cool. Repeat with the remaining dough.
Make Ahead Tip: The filling (Steps 1 & 2) can be made up to 2 days ahead and kept, covered, in the refrigerator. The dough (Step 3) can be made up to 1 day ahead. Store cookies in an airtight container for up to 3 days or freeze for up to 1 month.
Storage smarts: For long-term freezer storage, wrap your food in a layer of plastic wrap followed by a layer of foil. The plastic will help prevent freezer burn while the foil will help keep off-odors from seeping into the food.