I have an old box of Koopman's Oud-Hollandse Appelkaneel cake mix.
The recipe calls for you to whip some butter using a mixer before adding the cake mix and eggs. Then it should be beaten some more with the mixer. Presumably, this is to include air into the mix and assist in the leavening process.
However, I don't have a mixer. I do, however, have a handheld immersion blender. Would it be possible to use thus tool for this purpose? If so, should I add a bit of full-cream milk to the butter to help it whip, since it should still blend as long as it has a fat content above 30%?