Heat oven to 375°F. Line two rimmed baking sheets with parchment.
Stir together avocado, tomato, corn, red onion, 2 tsp lime juice, chili powder, half of garlic and ¼ tsp salt in a medium bowl.
On a clean counter top, place one sheet of pastry. Brush evenly with butter and top with another sheet, smoothing to flatten.
Brush second sheet with butter and top with a third sheet. Cut sheet along the long edge into 5 even strips. Cut each strip in half crosswise and half again to make 4, 3-inch squares. Repeat with remaining strips.
Fill one half of each square with 1 tbsp filling, leaving a ½-inch border.
Brush border with egg wash.
Fold pointed edge of unfilled half over to create a triangle and press firmly to adhere to egg washed edges.
Place on prepared sheet and brush triangles with egg wash.
Bake until golden, about 10 minutes. Repeat with remaining pastry sheets and filling.
Meanwhile, mix sour cream, cilantro, remaining lime juice, garlic and salt in a small bowl. Serve with pockets.
Notes
If you enjoy a little spice, stir ¼ tsp cayenne pepper or your favourite hot sauce into dipping sauce.
Phyllo pastry likes to dry out quickly. While brushing pastry with butter, keep not yet used sheets covered with clean, dampened kitchen towel.