Preheat oven to 375 F degrees.
Melt the 2 tbsp of butter in a skillet. Add onion and chicken and cook until the chicken is no longer pink. Season with salt and pepper. Add bell pepper and corn and continue cooking for a couple more minutes just until pepper softens a bit. Empty the chicken mixture into a bowl and set aside until it cools a bit.
Add feta cheese and green onions and stir.
For handling the phyllo dough, unroll it and lay one sheet flat on your work surface. For the remaining phyllo dough, cover it with a damp towel, so it won't dry out. Brush the sheet with melted butter and add another phyllo sheet over the first one.
Using a pizza cutter, cut the sheets lengthwise into 4 long pieces, about 2 inch strips. Place a heaping tbsp of the chicken mixture near 1 corner of the phyllo strip. Fold the end at an angle over the filling to form a triangle. Continue to fold the triangle along the strip until you reach the end, like folding a flag. Repeat this with the remaining phyllo and filling.
Place your spanakopita triangles on your baking sheets and brush with butter. Bake them for 20 to 30 minutes until the triangles are golden brown and crisp.