Spray the inside of your slow cooker with non-stick cooking spray and line the bottom with parchment paper.
In a large bowl, whisk together almond flour, coconut flour, sweetener, baking powder, and salt until well combined.
In a separate bowl, beat the eggs, then add melted butter, almond milk, and vanilla extract, mixing until smooth.
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.
Place a clean kitchen towel under the lid to catch condensation, then cook on low for 2.5 to 3 hours until edges pull away and a toothpick comes out with moist crumbs.
Turn off the slow cooker and let the cake cool for 15 minutes before carefully removing and slicing.