1. Add 1/2 inch water (about 2 cups) to slow cooker and place aluminium foil rack in bottom* (I used a little trivet that I had, but you can also make a little coil out of foil).  Make foil sling for 8 1/2 by 4 1/2-inch loaf plan by folding 2 long sheets of aluminium foil; first sheet should be 8 1/2 inches wide and second sheet should be 4 1/2 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan.  Push foil into corners and up sides of pan, smoothing foil flush to pan.  Lightly grease foil.
2. Microwave chocolate in large bowl at 50 percent power for 1 to 2 minutes. Stir, add butter, and continue to heat until melted, stirring once every 30 seconds; let cool slightly. In separate bowl, whisk flour, baking powder, and salt together. Whisk brown sugar, egg and yolk, sour cream, and vanilla into cooled chocolate mixture until well combined. Stir in flour mixture until just incorporated.
3. Scrape batter into prepared pan and smooth top.  Set brownies on rack in prepared slow cooker, cover and cook until toothpick inserted in center comes out with few moist crumbs attached, 3 to 4 hours on high (mine took about 2 1/2 hours).
4. Let brownies cool completely in pan on wire rack, 1 to 2 hours (I waited 30 minutes until I couldn’t take it anymore). Using ends of foil as handles, lift brownies out of pan and transfer to cutting board; discard foil. Cut brownies into squares and dust with confectioners’ sugar, if using. Serve.

*Making a foil rack: To make an aluminum foil rack to elevate your baking pan so water doesn’t seep in, loosely roll a 24 by 12 inch piece of foil into 1-inch cylinder. Then bend in sides to form oval ring that measures 8 inches long by 5 inches wide. After adding water to slow cooker, place foil rack in center, then place pan on top.