Add the sour cream, vegetable oil, eggs, and Guinness. Mix well.
Spray the slow cooker with a baking spray (or just coat with a little vegetable oil if you don't have baking spray). Add the batter to the slow cooker and cook on low for 2 to 2 1/2 hours (until the middle does not jiggle when you shake the slow cooker).
For the whipped cream (optional):
Place the heavy cream in a medium bowl. Beat with a hand mixer until the cream begins to thicken.
Add the sugar and Guinness, and continue to beat until the cream is stiffened.
Don't worry if the batter seems a little thin, that's normal.
Don't skip spraying the slow cooker with a non-stick spray or your cake will stick.
Most of the time I serve the cake right out of the slow cooker. To remove it to serve later, separate the cake from the sides by running a silicone spatula around the perimeter. At that point, I cut it in half to lift it from the slow cooker using the spatula and a pancake flipper (It's tough to lift the whole thing at once so I do one half at a time.)
I recommend making the whipped cream right before serving for best results. You can make it a few hours ahead of time, however. Leftover whipped cream can be stored in the refrigerator for a day or two. It will lose some of its fluff but it will still taste good!
This cake can be served by itself, with the whipped cream, or for a super-special dessert you can upgrade it with fresh cherries, cherry pie filling, and/or chocolate syrup. Yum!