Step 1Heat the oven to 325 degrees. In a medium saucepan, combine the sugar, water and lemon juice. Bring to a boil over medium-high heat, stirring frequently, then remove from heat and set aside to cool.
Step 2In a large bowl, sift together the baking powder with the flour. In a separate medium bowl, beat together the eggs, cooking oil and yogurt. Gently stir the liquid ingredients into the dry. Knead to a fairly soft but not sticky dough, adding a little water if necessary.
Step 3Divide the dough into 40 equal pieces (each will weigh about three-fourths of an ounce and will be about the size of a walnut in its shell). Roll out each one to the size of a small breakfast plate (about 5 inches in diameter), sprinkling with cornstarch to stop the dough from sticking to the counter or pastry board. Stack 20 of the pastry sheets, sprinkling cornstarch in between each layer. Roll out the stacked sheets at once until the dough is about 15 1/2 inches in diameter. (You will need to place it on a large rimmed baking sheet; if you do not have one that's big enough, divide the dough into 80 equal pieces and prepare 2 smaller trays of baklava.)
Step 4Lay the combined rolled sheets of dough on the baking sheet. Sprinkle the ground walnuts evenly over. Roll out the next 20 sheets of dough in the same way as the first batch and place on top of the walnuts. Using a sharp knife, cut the baklava into parallel slices about 2 inches apart, then rotate the baking sheet by 45 degrees and cut again into parallel slices.
Step 5Spoon the melted butter over the baklava. Bake until the top of the baklava is golden and crisp, 30 to 45 minutes. Remove and pour over the cooled syrup. Serve hot or cold.