Preheat oven to 350 degrees. Line an 8x8 baking dish with aluminum foil and grease pan (I used baking spray). This makes for easy cleanup. Set aside until needed.

In a microwave-safe bowl, melt your butter 20-30 seconds at a time and stir to prevent popping.

Pour butter into medium mixing bowl. Add first 7 ingredients (melted butter included) from the list above (flour, oats, sugar, brown sugar, vanilla, and salt). Stir well. Mixture should resemble sand with a few larger clumps. It will still be a bit dry.

Set aside 1 heaping cup of the crust mixture. This will be used as the crumble topping.

Pour the remaining crust mixture into the bottom of your baking dish. Using the back of a spoon or spatula, press firmly into an even layer, covering the entire bottom. This will ensure your bars will stay together after baking.

Using the bowl from the crust/crumble topping, combine blueberries, sugar, flour, and lemon juice. Stir until evenly mixed. Pour Blueberry mixture evenly over crust layer in your baking dish.

Using the reserved crust mixture, sprinkle crumb topping evenly all over the top of the blueberries.

Bake about 45-50 minutes (if you used frozen blueberries). Bake time may be 10-15 minutes less if you used fresh berries. Always start with a lower time and add more if needed. Bars should look set with a light golden top. Watch them carefully as the crumble topping can burn easily towards the end if your aren’t careful.

Let bars rest and cool at least 30-45 minutes or until well set. This will prevent them from falling apart. Cut into 12 bars. I recommend storing them in the fridge so they stay together better. They will keep well for about a week or so.