• Position a rack in the center of the oven and preheat it to 375°F. Grease and flour a 9-inch square baking pan.
  Butter and flour
  
• Beat the butter and sugar together with an electric mixer until light and fluffy, or about 3 minutes. Add the egg and vanilla and beat until blended.
  5 tbsp unsalted butter | 3/4 cup granulated sugar | 1 large egg | 1 tsp vanilla extract
  
• Combine the flour, baking powder, baking soda, and salt in a bowl, stirring to blend. Add approximately one-third of the dry mixture to the first mixture, using half of the buttermilk. Beat until smooth. Repeat this process with the remaining 1/3 of the dry mixture and the remaining buttermilk. Add the remaining flour mixture and beat just until smooth. Fold the blueberries into the batter.
  2 cups (255 grams ) all-purpose flour | 1 1/2 tsp baking powder | 1/4 tsp baking soda | 1/2 tsp salt | 1/2 cup buttermilk | 2 cups blueberries
• Spoon the batter into the prepared baking pan and spread gently.
  

Make the Topping

• Combine 4 tablespoons of unsalted butter with the brown sugar, 1/3 cup of flour, and cinnamon; blend with a fork or your fingers until crumbly. Sprinkle the topping mixture over the cake batter.
  4 tbsp butter | 1/2 cup brown sugar | 1/3 cup all-purpose flour | 1/2 tsp ground cinnamon
  
• Transfer the cake to the oven and bake for 25 to 35 minutes or until it springs back when lightly touched. A toothpick should come out clean when inserted into the center of the cake.
  
• Slice the cooled cake and serve on small plates with extra berries or a scoop of ice cream.