Greetings
what I use is "Natron" (Sodium carbonate). I suppose this is the baking soda available in every supermarket. Note I wrote Baking Soda. Not baking powder. That is some other stuff. The soda is some very fine "powder", like sand really. You mix it with a white glue/water mix to the consistency you need. When drying the mix will become more grainy, which is okay.
I would advise you to tint your carbonate/whiteglue/water mix with a drop of blue color. Not too much, a drop is sufficient. Snow (and ice) is not "only white". With a slight blue hue to your snow the bases will look spectacular. More so if, after plastering the base with the slightly colored mix, you sprinkle lightly some carb over the still wet mix.
Hope that helps
Jo