Can I swap out butter and milk for heavy cream in a ganache recipe? I want to make a ganache today, but the heavy cream I expected to use has gone bad and the local stores are all closed right now. My normal recipe is 1:1 ratio of cream to chocolate.
I know that I can't substitute butter and milk for whipped cream (e.g.) because it's not emulsified or anything and won't whip. However, will the melting and stirring involved let me get away with it for a ganache? Since cream is ~35% fat, am I right that 35% butter plus 65% milk would end up with a reasonable consistency?
Yes you can, some Portuguese/Brazilian ganache recipes actually don't use cream at all.
Cream (creme de leite) can be prepared this home-made way:

500 ml of fresh milk
1 egg yolk passed through the sieve
200 g butter
200 g hydrogenated vegetable fat (margarine)

In a pan, mix the sieved yolk and the milk, heat it slow so that the yolk does not cook and the mixture does not boil.
Turn off the heat and add the other two ingredients. As soon as the fat and butter have completely melted, beat it still hot for three minutes in the blender.
now you have your cream, continue finishing your ganache as usual...