In a microwave-safe bowl, melt 1 cup dark chocolate chips on medium power in 30-second intervals until just melted. Set aside.
In a large bowl, beat butter, brown sugar and granulated sugar until creamy. Add melted chocolate and mix until combined.
Add egg and vanilla extract and mix to combine well.
Add the flour, cocoa powder, baking powder, and salt mix until blended
Stir in remaining cup of chocolate chips.
Spread the dough into the prepared pan and smooth the top. Bake for 30 minutes. After 30 minutes, give the pan a little shake. If it jiggles in the middle, put the pan back in and try again every 5 minutes until there is no longer any jiggle.