To prepare filling, place potatoes in a saucepan; cover with water. Bring to a boil; reduce heat, and simmer 15 minutes or until tender. Drain. Combine potato, broth, dill, 1/2 teaspoon salt, pepper, and salmon in a large bowl; mash until well combined.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion; cook 3 minutes or until soft, stirring once. Stir onion into potato mixture. Set aside.
To prepare dough, lightly spoon 2 1/3 cups flour into dry measuring cups; level with a knife. Combine 2 1/3 cups flour, baking powder, and 1/4 teaspoon salt in a large bowl. Combine yogurt, 1/4 cup water, butter, and 1 egg in a medium bowl, stirring with a whisk. Make a well in center of flour mixture; add yogurt mixture, stirring until dough forms. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add 1 tablespoon flour to prevent dough from sticking to hands (dough will feel sticky). Cover dough, and let stand for 10 minutes.
Preheat oven to 375°.
Divide dough into 16 portions. Working with one portion at a time (cover remaining dough to prevent drying), roll each portion into a 5-inch square on a floured surface. Place 1/4 cup potato mixture in the center of dough. Fold dough over filling, pinching seam and ends to seal. Place knishes, seam sides down, on a baking sheet coated with cooking spray. Repeat procedure with remaining dough and filling.
Make a small cut in center of top of each knish. Combine remaining 1 tablespoon water and remaining egg in a small bowl, stirring with a whisk. Brush egg mixture over knish tops. Bake at 375° for 30 minutes or until golden.