I've tried making this one in the thermomix, but it was a flop so if you're thinking of giving it a go, try at your own risk!
Texas Sheet Cake225g butter
1 cup water
4 Tbs cocoa
2 cup SR flour
2 cup sugar
1/2 tsp salt
1 cup milk
1 Tbs vinegar
1 tsp bicarb
1 Tbs vanilla
Place butter, water and cocoa in a saucepan. Mix over heat until melted and boiling.
Place flour, sugar and salt in a large bowl.
Combine eggs, milk, vanilla, bicarb and vinegar in a small bowl and whisk well.
Mix all ingredients together in the large bowl.
Pour mixture into greased cake pan and cook in a 200 degree celsius oven for 30-40 minutes.
For the icing, I put a bit of softened butter (maybe 4 Tbs?) into a bowl then add icing sugar and cocoa and milk to make enough. Mix with a spoon and spread over cooled cake.