First, use aluminum foil to make a "collar" to go inside your 6-quart slow cooker - this will keep the edges of your brownies from getting burned while the middle finishes baking. There is no perfect way to do this - just use a few sheets of foil, folded several times, and line the inside of your slow cooker. (See picture in post).
Once you have the inside lined with a ring of foil, spray the whole inside with nonstick spray. Place a sheet of parchment paper in the bottom, inside the foil collar. (See picture)
In a small bowl, whisk together flour, cocoa, baking powder, and salt. Set aside.
Place butter and chocolate in a medium microwave-safe bowl and microwave in 30-second increments, stirring after each, until chocolate is melted and smooth. Add sugar; stir to combine. Stir in eggs and vanilla until smooth.
Add flour mixture and chocolate chips and stir just until combined (do not overmix). Transfer batter to slow cooker.
Cover and cook on low for 3 hours. Uncover, turn the slow cooker off and allow to cool in the slow cooker for 1 hour. Remove (by lifting the parchment out) and allow to set completely before cutting into bars and serving.
*Other sizes of slow cookers make work for these brownies but you'll need to keep an eye on the cooking time.