Can I swap out butter and milk for heavy cream in a ganache recipe?
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...