Set crockpot at High 20 minutes before beginning. Fit in liner. butter and flour 8-cup-capacity baking tin. Mix flour, cream of tartar, and salt; set aside. Cream butter, then add sugar, vanilla, and mace. Beat until fluffy. Add eggs, one at a time, beating 1 minute after each. Blend in flour mixture; do not overmix. Spread in tin, and put on lid or lay terry towel over it. Put in crockpot, cover and bake 3 hours or until cake has begun to pull from sides. Uncover. Let stand in tin on cake rack about 10 minutes, then turn out to cool. Dust with confectioners sugar, slice and serve.
Note: Two secrets for cake-baking in crockpot: use less than 1 c sugar for every 2 c flour. And use a liner. (Kitchen parchment to be used as a liner is available in most department store housewares sections. It's reusable and keeps food from sticking. Substitute buttered or oiled brown shopping bag paper, if you want.)