Preheat the oven to 350°F and butter a donut pan well, getting into every groove.
In a measuring cup, combine the buttermilk, apple cider, and vanilla extract.
In a separate bowl, whisk together the flour, salt, cinnamon, nutmeg, baking powder, and orange zest.
In a mixing bowl with the paddle attachment, cream the butter and both sugars until light and combined.
Add the eggs one at a time, beating well after each addition.
With the mixer on low, alternate adding the wet and dry ingredients until just combined. Scrape down the sides and bottom of the bowl.
Spoon half the batter into a piping bag, or a zip-top bag with about 1/2 inch snipped off the corner. Pipe into the prepared pan, filling each well about two thirds full.
Bake for 15 to 18 minutes, checking at 15. The donuts are done when they spring back to the touch.
Combine the granulated sugar, cinnamon, and cardamom in a shallow bowl. Melt the butter in another.
While the donuts are still warm, brush the top and bottom of one donut with melted butter, then dip both sides in the cinnamon cardamom sugar. Repeat one at a time with the rest.
Then, dip both sides in the sugar and spice mixture.
Notes
Please don't leave out the cardamom in the dipping sugar. It's what makes these different from every other cider donut recipe.
Dip the donuts one at a time. If you butter the whole batch first, the butter soaks in, and the sugar won't stick.
No donut pan? Use a greased 12-cup muffin tin, filled two-thirds, at 350°F for 15 to 18 minutes. For donut holes, use a mini muffin tin and bake for 8 to 10 minutes.
Want a stronger apple flavor? Simmer 2 cups of apple cider down to 1 cup, cool completely, and use that in place of the unreduced cider.
Apple juice works in a pinch, but use unsweetened. If yours is sweetened, reduce the granulated sugar in the batter slightly.
Make the batter the night before and refrigerate for fresh donuts in the morning.
To freeze, coat the donuts first, then wrap in plastic and store in an airtight container for up to 2 months.