Baking – What other vegetables can be used in cake/muffin baking

Once you've got your wet ingredients in there the timer is running. The chemical rising action only lasts a little while, and you want as much of that as possible to happen while the muffins are in the oven. If you wait past that window your muffins aren't going to rise very well, if at all, and I think you'd definitely run into that with freezing the mixture.

Even if freezing the batter would stop the chemical reaction (which I seriously doubt), you wouldn't be able to make use of that fact. Using a conventional freezer, the cooling process is going to be fairly gradual. Your batter is going to be at an active temperature for quite a while before eventually reaching the mythical non-reactive temperature--far longer than the chemical rising action lasts. And then as you thaw the batter you have another lengthy period where your chemical clock is ticking, long before you can get it in the oven.

Buttercream is pretty simple, start with room temperature butter, beat it until it's white, slowly add icing sugar, add small amounts of milk as needed to keep the consistency right.

For each 1/4 cup (1/2 stick) of butter, use about 1 1/2 cups of icing sugar (sifted) and 3 Tbsp of milk. You can do it in a mixer, a food processor, or just use a bowl and a whisk. I used a whisk a few days ago when I realized my large stand mixer is overkill for the small amount of icing I wanted and I found that microwaving the icing for about 10 seconds helped me stir in the sugar when it was getting too thick.