Spray a donut pan with a non-stick baking spray. Set aside.: The smell of non stick spray is faint and reassuring, and preparing the pan first saves you from scrambling later. Coating every well thoroughly ensures the donuts release cleanly, which matters for presentation. A common misstep is skimping on spray in the crevices, causing the donut edges to tear when unmolding; take an extra moment to cover each cup well.
Preheat oven to 425 degrees.: Preheating fills the kitchen with a warm, slightly yeasty scent and establishes the oven environment needed for quick rise. Starting with a fully heated oven gives the donuts an initial lift so they set with a tender crumb and defined shape. If you skip preheating the donuts can bake unevenly, producing dense spots, so wait until the temperature is stable.
In a large mixing bowl, whisk together flour, sugar, baking powder and salt.: The dry mix will look uniform and pale, with soft dustings of sugar and tiny specks of leavening. Whisking aerates the flour slightly and distributes baking powder evenly, which is essential for consistent rise. Sift if the flour is lumpy, and avoid overworking the mixture, as that can develop gluten and yield tougher donuts.
In another mixing bowl, whisk together eggs, buttermilk, vanilla and oil.: The wet mixture should be smooth and slightly glossy, and the aroma of vanilla will float up pleasantly. Whisking until homogeneous helps the batter come together with fewer streaks. A typical error is not fully combining the liquids, which creates pockets of unmixed egg or oil in the final batter.
Add the wet ingredients to the dry ingredients and stir until combined. Do not over-mix. The batter should be lumpy.: You will feel a satisfying resistance as the batter comes together, and small lumps are desirable to keep the texture tender. Stir only until the ingredients are just incorporated; over mixing activates gluten and will make the donuts tough. If you notice a glossy, elastic batter, you have mixed too much, and the crumb may be chewier than intended.
Place the donut batter into a ziploc bag. Snip one corner of the bag off. Pipe out the batter onto the donut pan, filling the cups 2/3 full.: Piping produces neat, even donuts and reduces mess. The batter should flow with a slight resistance as you pipe, and the cups should look mostly full but not overflowing. Filling too little yields oddly shaped donuts, while overfilling causes spilling and uneven baking, so aim for that two thirds mark for consistent size.
Bake the donuts for 9 minutes.: As they bake, the kitchen fills with warm, sweet notes and the tops will turn faintly golden. The donuts should spring back gently when touched and a toothpick inserted near the center should come out clean. Leaving them in too long can dry them out and darken the edges, so watch the oven closely around the end of the time.
Cool for 10 minutes in the pan, then transfer onto a cooling rack to cool completely.: Cooling in the pan for a short while lets the structure set so the donuts unmold cleanly, then moving them to a rack prevents steaming and sogginess. You should notice a slight give when they are warm, and by the time they are cool the texture firms to a tender cake feel. Rushing the transfer while still very hot can cause breaks, and letting them sit too long in the pan can trap moisture.
Pop the popcorn according to the instructions on the package.: Freshly popped corn has a light, toasty aroma and satisfying crispness, which contrasts beautifully with the donuts. Use plain kernels so the white chocolate and sprinkles shine. A common pitfall is overcooking the popcorn or leaving unpopped kernels; stay attentive during popping to avoid burnt corn.
Melt white chocolate in a heat-resistant dish.: Melt the white chocolate chips gently until glossy and smooth, stirring frequently to prevent scorching. The aroma is sweet and creamy, and the texture should be pourable but not runny. If you overheat the chocolate it can seize, becoming grainy, so use low heat and short bursts in the microwave or a double boiler for control.
Spread a layer of white chocolate over the top of the donut. Top with popcorn. You may have to hold it in place for few seconds until the chocolate "glues" it to the donut.: When you press the popcorn into the melted chocolate, the scent of warm chocolate and toasted corn becomes intoxicating. Hold each kernel in place briefly so the coating sets around it, creating a secure bond. If the chocolate cools too much it will not adhere, and if you wait too long the popcorn will slide off, so work while the chocolate is glossy.
Sprinkle donut with sprinkles and drizzle with more white chocolate.: The bright colors from the funfetti sprinkles bring immediate cheer, and an extra drizzle of white chocolate seals everything together visually and texturally. The drizzle should be smooth and ribbon like, adding shine and another layer of sweetness. A heavy hand with sprinkles can overwhelm the other elements, so scatter them thoughtfully.
Repeat for the remaining donuts.: As you assemble the rest, you will notice your rhythm improving and the platter becoming increasingly impressive. Keep an eye on chocolate temperature, reheating gently if it begins to thicken, and enjoy the tactile act of finishing each donut. The main mistake to avoid now is working too slowly and letting the chocolate set between steps, which makes assembly fiddly.