In a medium size bowl, mix together the rolled oats, brown sugar, flour, and cinnamon. Cut in the butter with a pastry cutter or two knives until the mixture is crumbly and resembles bread crumbs.

Set the crumb topping aside. (If it's a warm day, I recommend popping the bowl in the fridge!)

Preheat the oven to 375 F.

Grease a 8x8" or 9x9" square cake pan and sprinkle with flour. Line the cake pan with parchment paper. (See note)

In a large bowl, beat together the butter and sugar until light and fluffy. Then beat in the egg and vanilla. Stir in the lemon zest.

In a medium size bowl, stir together the flour and baking powder.

Measure out the milk.

To the large bowl of wet ingredients, add 1/3 of the dry ingredients, and mix until just moistened. Then add 1/2 of the milk and mix until incorporated. Again, add 1/3 of the dry, 1/2 the milk, and the last 1/3 of the dry, mixing between each step. The batter will be very thick.

Gently fold in the blueberries.

Pour the batter into your prepared pan, smoothing it flat. Sprinkle the crumb topping evenly over the cake.

Bake cake for 40-45 minutes, until a toothpick inserted in the center of the cake comes out clean.

Cool the cake in the pan, and then carefully lift out using the edges of the parchment paper. Cut into 16 squares. Serve.