• Preheat the oven to 350℉/176℃ and line a 9x9 baking pan with parchment paper. Set aside.

For the streusel

• In a bowl, add the butter, white sugar, brown sugar, cinnamon, and flour. Mix together with your hands to form a buttery sugar streusel and set aside in the fridge as the cake batter is being made.
  3/4 cups all-purpose flour, 1/3 cup white sugar, 1/4 cup light brown sugar, 5 tbsp butter, pinch of cinnamon

For the cake batter

• In a bowl, add the flour, baking powder, and salt. Stir to combine and set aside.
  2 cups all-purpose flour, 2 tsps baking powder, 1/4 tsp salt
• Into a large bowl, add the butter, white sugar, and lemon zest. Mix on medium-high speed for 2-3 minutes until light and fluffy, scraping the sides of the bowl as needed.
  1/3 cup unsalted butter, 1 tsp lemon zest, 3/4 cups white sugar
• Add in the egg and continue mixing for another minute until well combined, scraping the sides of the bowl as needed.
  1 large egg
• Now we're going to alternate adding in the dry and wet ingredients. Starting off with the dry, add in about 1/3 of it, and mix on medium speed just until combined.
  1/2 cup milk, 1 tbsp vanilla
• Add in about 1/2 of the wet ingredients, and mix until just combined. Continue this process until a thick batter has formed. It should go dry-wet-dry-wet-dry.
• Toss the blueberries in 1 tbsp of flour and evenly coat them, then add them into the bowl and gently mix with a spatula. Try not to add the extra flour at the bottom of the bowl in.
  2 cups blueberries