1. Preheat the oven to 425F and grease your doughnut pan with nonstick spray or oil. 

2. In a electric mixer, combine butter, oil, and sugar and beat until smooth and fluffy.

3. Add in eggs, vanilla, lemon zest, milk, flour, baking powder, baking soda, and salt. Mix together until just combined. 

4. Add in blueberries and use a rubber spatula to stir in blueberries. The batter will be thick. 

5. Spoon your batter into a large piping bag (or makeshift piping bag using a plastic Ziploc bag), cut off the end of your bag so it is large enough to get blueberries through the opening (1/2-3/4" wide). Pipe the batter in once around each doughnut hole. The batter shouldn't fill the doughnut hole completely as they'll expand when baking. 

6. Bake the doughnuts for approximately 7-8 minutes or until a toothpick comes out clean. Let cool in the pan for 10 minutes before placing them on a cooling rack to cool completely. 

Making the Glaze

1. In a medium-large bowl (wide is better than deep), combine sugar, zest, and lemon juice. Stir together. You want your glaze to be thin enough to evenly coat your doughnuts but thick enough to dry and become hard to the touch. If you need to, you may add more lemon juice if the glaze is too thin. 

2. When the doughnuts are completely cool, dip the tops of the doughnuts into the glaze and turn them right side up again so the glaze runs down the sides. Set aside and let the glaze set. Or not, thats cool too! 

3. Notes: If you are making these ahead of time for an event, make the doughnuts, let them cool and store them in a airtight container. Make the glaze and glaze the doughnuts a few hours before you serve them. These doughnuts can be stored in an airtight container with the glaze on them as well.