Preheat the oven to 350 degrees F and coat a donut pan with nonstick spray.
Half the blueberries with kitchen scissors.
Add the flour, sugar, cocoa powder, baking powder, and salt to a medium bowl and stir to combine.
Add the milk, butter, egg, and vanilla extract. Stir gently. Fold in the berries and stir gently to just combine. (This batter is thick and it may look too thick, but keep gently stirring to bring it together.)
Portion out into the donut pan, dividing the batter among each of the 6 rounds and spreading so the batter is somewhat evenly distributed.
Bake for 14-16 minutes or until a cake tester inserted into the center comes out cleanly.
Remove from the oven and let cool in the pan for 2 minutes. Run a paring knife around the edges of each doughnut and carefully flip onto a wire rack to cool fully.
Optional drizzle: Add the chocolate chips to a heat-safe bowl. Microwave in 30 second increments, stirring in between until melted. (Mine usually take 1 minute.) Drizzle over the donuts with a spoon.
Notes
Store cooled donuts in an airtight container at room temperature for up to 3 days.
Melt chocolate chips and drizzle over for extra sweetness.
Dairy-free: Use nondairy milk and neutral oil in place of the milk and butter.
Gluten-free: Use cup-for-cup gluten-free flour blend.
To make these with frozen blueberries, let thaw partly at room temperature. (This will help prevent the batter from clumping together.)
This batter is thick by design—the berries add moisture when they're baked into the donuts.