Combine the lemongrass and ⅓ cup sugar in a small bowl and rub with your fingers until the sugar is infused with the lemongrass, 1 minute. Strain the sugar through a fine-mesh strainer into a medium bowl and reserve. (Use the lemongrass for tea or in your next bubble bath!
Combine the pudding, lime zest and juice, and ¼ teaspoon of the salt in a small bowl; let sit on the counter until ready to use (or make and refrigerate up to 24 hours in advance.
Whisk together the flour, 2 tablespoons sugar, baking powder, and ¾ teaspoon salt in a medium bowl. Add the butter and stir with a wooden spoon until the butter is coated with the flour and mostly absorbed by it, 20-30 seconds. Gently stir in the buttermilk until a shaggy ball of dough forms, add 1-2 tablespoons more buttermilk if the dough is too dry.
Generously flour a clean work surface, dump the dough on top of the flour and gently knead the dough until unified and less sticky, 30 seconds. Pat the dough into a six-inch square and use a knife, pizza cutter, or pastry scraper to cut the dough into 16 equal-sized pieces. Use lightly floured hands to roll the pieces into rounds.
Arrange a rack on top of a sheet pan. Heat three inches of oil in the Lowkey Lavender Braiser or other heavy-bottomed pot to 360°F (use a candy thermometer, or drop a tiny piece of dough into the oil; if it begins to sizzle and brown on contact but doesn’t burn, you’re ready). Gently lower half the donut holes into the oil and fry, flipping once, until golden and crisp, 1 ½ to 2 minutes per side. Drain briefly on the rack, then toss in the lemongrass sugar. Transfer to a serving plate and repeat with remaining dough. Serve the donut holes with the key lime pie dipping sauce.
