Grease a Casserole Crock™ or other large Crock-Pot® slow cooker with non-stick spray.
Melt chocolate and butter together in a small bowl, using 15 second increments in the microwave (stir every 15 seconds until smooth)
Set mixture aside to cool while you prep other ingredients
Whisk sugar, eggs, and vanilla in a medium bowl until well blended
Add chocolate mixture and stir until well combined and smooth.
Sift flour and baking soda together and pour into chocolate mixture.
Gently stir just until combined – do not over mix!
Pour brownie mixture into the greased Crock-Pot® slow cooker and even out with a spatula.
Cover and Cook on LOW for 4 to 5 hours or until done.
Remove lid and cool completely before cutting.
Cut into bars just prior to serving. Sprinkle with sifted powdered sugar for garnish.