Preheat oven to 325°F. Lightly coat a 13" x 9" baking pan with the oil spray. Cut a 13" x 13" piece of parchment paper to fit the bottom and up 2 of the 4 sides of the pan (the parchment paper will later be used as handles to lift the square out of the pan).
Combine the oats, flour, cinnamon, salt and baking powder in a bowl. Place the butter and brown sugar in a second bowl and beat until well combined. Beat in the eggs and vanilla. Add the oat mixture to the wet ingredients and mix until well combined.
Press the oatmeal cookie dough evenly into the pan. Top the dough with the Halloween chocolate bars and candy.
Bake the cookie square 25 minutes. Remove from oven and cool completely. Cut around the edges of the cookie square and then lift it out of the pan. Cut into squares and serve. These cookie squares freeze well.
Note: Use a mix of mini chocolate bars, cut into 1/2" cubes and other treats, such as Smarties or M&M'S, on the square.