In a 4-liter bowl microwave-proof, combine butter and cream. Cook on “High” for 1 minute. Stir in brown sugar, icing sugar, flour and cream of tartare. Transfer back to the microwave and cook on “High” for 2 minutes, stirring half way through cooking. Add vanilla and stir again. Cook on “High” for 4 to 4 ½ minutes, stirring half way through.
To know if it’s ready, take a very small amount of butter & cream fudge and drop it in cold water. If it forms a small ball and seizes then it’s ready otherwise, return it to the microwave for another 30 seconds. Add chopped walnuts, stir quickly and transfer to a greased bowl 9 x 4 (loaf pan) or silicone square mold. Let it cool for 30 minutes before chilling in the fridge until set and firm, about 2 hours. Cut into squares and serve.