In a large microwave-safe bowl, melt the butter for the lemon curd. Add lemon juice, eggs, sugar, and lemon zest, and whisk until combined.

Microwave the mixture for 1 minute, whisk thoroughly, then continue microwaving for 30 seconds at a time, whisking well after each heating, until thick enough to coat the back of a spoon. Allow to cool and refrigerate until ready to use.

In a medium bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.

In another medium bowl, whisk buttermilk, egg, greek yogurt, and vanilla until combined. Add melted butter and whisk until fully incorporated.

Add dry ingredients to wet ingredients and fold until just combined. Gently fold in blueberries. Cover and chill in the fridge for at least 30 minutes.

Fill a large pot with oil to about 2 inches deep. Heat the oil to 350F-370F.

Line a wire cooling rack with paper towels.

Scoop dough using a small cookie scoop and drop into the hot oil, frying 6-8 donut holes at a time for 2-4 minutes until golden. Use a fork to flip during frying.

Remove donut holes with a slotted spoon and place on the paper towel-lined rack. Repeat with remaining dough.

To make the glaze, whisk together powdered sugar, buttermilk, and vanilla in a medium bowl until smooth, adjusting thickness as needed.

Dunk each donut hole in the glaze and toss to coat. Set on a cooling rack until glaze sets.

Transfer lemon curd to a piping bag. Insert the filling tip into the donut holes and squeeze in the lemon curd.