Preheat oven to 350°F (175°C). Grease a 9x9-inch baking dish or line with parchment paper.
Whisk flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
Cream butter, sugar, and lemon zest in a large bowl until fluffy, 2–3 minutes. Beat in eggs, vanilla, lemon juice, and sour cream until smooth.
Gradually add dry ingredients, mixing until just combined. Fold in flour-tossed blueberries.
Pour batter into the pan and smooth the top.
For the streusel, mix flour, sugars, and cinnamon. Cut in cold butter until crumbly. Sprinkle over batter.
Bake for 40–50 minutes, or until a toothpick comes out clean. Tent with foil if browning too quickly.
Optional: Mix lemon juice and sugar for syrup. Poke holes in warm cake and drizzle syrup over. Cool for 15 minutes.
Whisk powdered sugar and lemon juice for glaze. Drizzle over cooled cake.
Video
Notes
Store at room temperature for 2 days, refrigerate for 5 days, or freeze for 3 months.Use fresh blueberries for best results, or frozen (unthawed) tossed with flour.