Preheat oven to 350 degrees. Grease 2 6-slot doughnut pans with butter.
In a large bowl, whip the butter and sugar until combined. Beat in the eggs, one at a time, until incorporated, then the vanilla and yogurt. Scrape down the bowl as needed with a spatula. Add the flour, baking powder, baking soda, and salt. With the mixer running, stream in 1/4 cup of the buttermilk. If the batter seems too thick, add the additional tablespoon to reach the right consistency. Fold in the funfetti sprinkles. Do not overmix.
Transfer the batter into a piping back, or a ziptop bag with a corner snipped out. Pipe into the prepared doughnut pans, filling 2/3 of the way to the top.
Bake for 10-12 minutes until an inserted toothpick emerges clean, or with a few moist crumbs. Cool in the pan for 5 minutes, then run a knife around the edges of each doughnut to loosen and transfer to a wire rack. Cool completely.
When ready to decorate, add the chocolate and coconut oil to a microwave-safe bowl. Heat for 1 minute. Stir until the bowl is no longer warm and the chocolate is free of lumps. If needed, heat an additional 10-20 seconds. Let cool 5 minutes.
Lay a piece of parchment beneath the wire rack. Dunk one side of each doughnut into the chocolate, letting excess drip back into the bowl. Sprinkle the iced side with funfetti sprinkles. Repeat with remaining doughnuts. Serve immediately, or cover and refrigerate for up to 5 days.