Preheat oven to 375°F and grease a 9" square pan.
Streusel Combine brown sugar, flour, cinnamon and salt in a small bowl. Using fingers, mix until no big lumps of brown sugar remain. Cut in butter. You want this in a crumbly state. Refrigerate until ready to use.

Cake In a medium bowl and using electric mixer, beat butter and sugar until creamy, around 2 minutes. Add eggs one at a time, scraping down sides of bowl each time so everything is worked in. Beat in vanilla and lemon zest.
Sift together flour, baking powder and salt. Gradually add flour
mixture to wet mixture, alternating with milk. Beat on low speed when combining. Use a spatula to lightly fold in blueberries.
Pour batter into pan. Add cold streusel topping over top of batter, covering it completely.
Bake 35-45 min. (start checking for doneness at 25-30 min. as oven temperatures vary). Test doneness with toothpick in middle of cake – it should come out clean.