Whisk flour, brown sugar, and salt in a large bowl. Stir in oil until the mixture is crumbly.
Stir baking powder, baking soda, and cinnamon into the flour mixture with a wooden spoon or spatula. Place milk, oil, eggs, and vinegar in a measuring cup and whisk until the eggs are beaten, then add to the flour mixture and stir until combined (mixture may be slightly lumpy).
Spray a 5-7Qt slow cooker with nonstick cooking spray, or line with a slow cooker liner (sprayed with cooking spray). Pour batter into crock pot.
Sprinkle nuts over the top of the cake batter.
Place a large paper towel over the top of the crock pot insert, then place the lid on top. Cook on HIGH for 1 1/2 to 2 1/2 hours (mine took 2), or until a toothpick comes out clean around the edges. The center might be a little under done on top (but it’ll be done underneath).
Serve warm straight from the crockpot or store in an airtight container for up to 3 days. For easy serving, use the slow cooker liner. That way you can just pick the whole liner out, peel it off, and serve the cake.
To bake in a traditional oven: use a 9x13” pan sprayed with nonstick cooking spray and bake at 350° for approximately 35-45 minutes.