3/4 cup (75 g) walnuts or pecans, toasted and chopped (optional)
Butter the inside of the slow cooker. Line the bottom with parchment paper, letting it extend up the sides.
In a bowl placed over a pot of simmering water or in the microwave oven, melt the chocolate with the butter. Remove the bowl from the heat and let cool. Whisk in the sugar. Whisk in the eggs, vanilla and salt. Add the flour and stir for about 2 minutes. Stir in the nuts, if using. Transfer to the slow cooker.
Cover and cook on Low for about 2 hours and 15 minutes. Cooking time may vary from one slow cooker to another. The edges of the brownies must be thoroughly cooked while the centre should be softer. Remove the inside container from the slow cooker. Let cool for 1 hour with the lid on. Remove the lid. Serve warm or let cool completely. Run a thin blade around the brownie to unmould.