In a large bowl add the spice cake mix, eggs, oil, water, shredded carrots, walnuts and vanilla extract. Mix until combined, don't overmix, some lumps are fine when using a cake mix. You can use a hand held mixer or a spatula.
Spray the slow cooker with non stick spray. Pour in the batter, spread it out evenly. Cook on high for 2 hours or until set in the middle.
When the cooking time is done, remove the ceramic insert from the heating element onto a cooling rack. Let the cake cool (uncovered) for at least 2 hours before frosting.
When the cake is cooled, start making the frosting. You can use a stand mixer, or a handheld electric mixer. Blend together the cream cheese and butter until combined. Add the powdered sugar, vanilla and milk. Blend until smooth, scrape down the sides of the bowl and blend again.
Spread the frosting on the cooled cake. Sprinkle with reserved nuts (and or royal icing carrots - from amazon). Slice and serve.
Notes
Place any remaining cake in an airtight container and keep it on the countertop or store it in the fridge for up to 3 days.
I bought the royal icing carrots from amazon, they are optional. You can sprinke the cake with nuts instead.