Ombre Brownies

1 cup butter, divided
1 3/4 cup light brown sugar, divided
3 large eggs, divided
2 teaspoons vanilla , divided

1/2 teaspoon salt, divided

1 3/4 cups all-purpose flour, divided

1 cup milk chocolate chips

1/4 teaspoon baking powder

1 cup dark chocolate chips

1/4 cup heavy cream

Put oven rack in middle position and preheat oven to 350°F.  Spray a 9-inch square baking pan with nonstick cooking spray.

Place 1/2 cup butter in a medium heavy saucepan and heat over low heat until melted.  Remove from heat.  Stir in 1 cup brown sugar until mixed.  Stir in 1 egg, 1/4 teaspoon salt, and 1 teaspoon vanilla until mixed.  Stir in 1 cup flour.  Spread evenly into prepared pan.  

In another medium heavy saucepan, place the remaining 1/2 cup butter and the milk chocolate chips over low heat until melted, stirring constantly.  Remove from heat.   Stir in remaining brown sugar, vanilla, and salt.  Stir remaining two eggs in 1 at a time, beating well after each addition, until mixture is glossy and smooth.

Whisk the baking powder and remaining flour together, then stir into chocolate mixture. Carefully spread batter over the blondie layer in the pan, then bake until a wooden pick or skewer comes out with a few moist crumbs adhering, 25 to 30 minutes.  Let cool completely on wire rack.

Place the dark chocolate chips in a small heavy saucepan along with the heavy cream and heat over low heat.  Whisk until melted and smooth.  Beat in 1/2 cup of powdered sugar until smooth and fluffy.  Frost brownies with ganache, then cut into squares.