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

Microwave the mixture for 1 minute, whisk, then microwave for an additional 30 seconds. Repeat until thick enough to coat the back of a spoon. Refrigerate until ready to use.

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

In a separate bowl, whisk together the buttermilk, egg, Greek yogurt, and vanilla. Add the melted butter and whisk until incorporated.

Fold the dry ingredients into the wet ingredients until just combined. Gently fold in the blueberries. Chill the dough in the fridge for at least 30 minutes.

Heat avocado oil in a pot to 350F-370F. Line a wire cooling rack with paper towels.

Scoop dough into the hot oil, frying 6-8 donut holes at a time for 2-4 minutes until golden. Use a slotted spoon to remove and transfer to the cooling rack.

Whisk together all glaze ingredients until smooth. Adjust consistency as needed.

Dunk each donut hole into the glaze and set on a cooling rack to let the glaze set.

Transfer lemon curd to a piping bag. Inject the curd into each donut hole, or dunk into the curd to fill.

Serve and enjoy.