1Heat oven to 350°F. Spray 13x9-inch pan with cooking spray.
2In large bowl, mix oats, 3/4 cup flour, the brown sugar and baking soda. Stir in melted 3/4 cup butter. Reserve 3/4 cup oat mixture for topping. Press remaining oat mixture in pan. Bake 10 minutes. Cool 5 minutes.
3Meanwhile, in 3-quart saucepan, heat chocolate and 2/3 cup butter over low heat, stirring occasionally, until melted; remove from heat. Stir in granulated sugar, vanilla and eggs. Stir in 1 1/4 cups flour, the baking powder and salt.
4Spread batter over baked base. Sprinkle with reserved oat mixture. Bake about 30 minutes or until center is set and oat mixture turnes golden brown (do not overbake). Cool completely, about 2 hours. Cut into 8 rows by 6 rows.