How to Make It
Drain the yogurt in a coffee filter in the refrigerator for at least 3 hours or overnight.
Preheat the oven to 375°. In a small saucepan, melt the butter. Let cool slightly, then pour into a small bowl, leaving the milk solids behind. Stir in the chile powder, cumin, coriander and cayenne.
Cut the phyllo sheets in half crosswise. Working with 3 sheets at a time and keeping the rest covered with plastic wrap and a kitchen towel, brush the sheets with some of the spiced butter and stack them in three separate 3-layer piles; discard the remaining phyllo. Trim any ragged edges. Cut 1 of the stacks into six 3 1/2 -inch squares. Transfer all the phyllo to 2 baking sheets, re-forming the cut rectangle, and bake for about 10 minutes, or until golden; let cool.
In a blender, puree the yogurt with the cilantro, chives and mint. Transfer to a small bowl and stir in the radish matchsticks, onion and jalapeño. Whip the cream until it holds firm peaks and fold it into the herbed yogurt; season with salt. Reserve 1/2 cup of the yogurt.
Set 1 of the large phyllo stacks on a work surface and top with half of the salmon and half of the herbed yogurt. Cover with the second large phyllo stack and the remaining salmon and yogurt. Top with the phyllo squares, re-forming the rectangle. Using a very sharp knife, cut the napoleons, between the squares, wiping the knife blade after each cut. Set each napoleon on a plate, top with a dollop of the reserved yogurt, garnish with radish slices and serve.
Make Ahead: The recipe can be prepared through Step 4 up to 1 day ahead. Wrap the phyllo on the 2 baking sheets tightly with plastic and store at room temperature. Refrigerate the herbed yogurt. Recrisp the phyllo in a 325° oven before proceeding if necessary.
Serve With: A mesclun salad.