Heat oven to 350 degrees F. Coat an 8 x 8 x 2-inch baking pan with nonstick cooking spray; set aside.
In a medium-size bowl, whisk together flour, baking powder and salt. Stir in milk, eggs, spinach and cheese until just combined. Pour batter into prepared baking pan and bake at 350 degrees F for 45 minutes or until lightly browned.
Remove from oven and let cool on rack for 15 minutes. Cut into squares, then cut squares into triangles and serve immediately.