In a large microwave-safe bowl, melt the butter. Add the lemon juice, eggs, sugar, and lemon zest. Whisk until combined. Microwave for 1 minute, then whisk. Repeat this for 4-6 cycles until thickened. Cool and refrigerate.

In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt. In another bowl, whisk buttermilk, egg, greek yogurt, and vanilla. Add melted butter and whisk until combined. Add dry ingredients to wet and fold until just combined. Gently fold in blueberries. Cover and refrigerate for at least 30 minutes.

Heat about 2 inches of oil in a heavy-bottomed pot to 350F-370F. Line a wire cooling rack with paper towels. Use a small cookie scoop to drop dough into hot oil. Fry 6-8 donut holes at a time for 2-4 minutes until golden. Use a slotted spoon to transfer to the cooling rack. Repeat until all dough is fried.

Whisk together glaze ingredients until smooth. Dunk each donut hole in the glaze to coat. Set on a cooling rack to let the glaze set. Fill a piping bag with lemon curd and fill each donut hole.