Line your slow cooker with parchment paper, leaving enough overhang on the sides to lift the brownies out later.
In a large bowl, whisk together the almond flour, cocoa powder, sugar-free sweetener, baking powder, and salt.
In a separate bowl, combine the melted dark chocolate, melted butter, eggs, and vanilla extract, whisking until smooth and well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined, being careful not to overmix.
Pour the batter into the prepared slow cooker and spread evenly.
Cover with a kitchen towel under the lid to catch condensation, then cook on high for 2 hours or until the edges are set but the center is still slightly soft.
Turn off the slow cooker and let the brownies cool for 30 minutes before lifting them out using the parchment paper and cutting into squares.