Preheat oven to 160°C. Grease and line the base and sides of a 20cm spring form cake pan.
Place butter into the Thermomix bowl, chop 10 sec/speed 5. Add Guinness and cook 2 min/100°C/speed 2.
Add cocoa and cook 3 min/100°C/speed 3. Set aside to cool slightly.
Into the empty bowl, place eggs, creme fraiche, vanilla, baking powder, sugar and salt. Combine 1 min/speed 5. Add the Guinness mix and combine 5 sec/speed 5.
Add flour 20 sec/Interval/dough setting. Finish combining with spatula. Pour into prepared tin and bake for 1 hour or until cooked through. Leave in tin for 10 minutes, then tip out onto a wire cooling rack. Cool completely before frosting.
To make frosting, place all ingredients into the Thermomix bowl and combine 20 sec/speed 5. Scrape bowl and repeat.
Spread frosting on the cake and enjoy!