Preheat the oven to 180C/350F and grease and line an 8x8 inch square pan. I use Wilton Bake Easy and grease proof paper.

Place butter and sugar into a large bowl, or the bowl of your stand mixer and beat until light and fluffy, around 2 minutes on medium-high speed.

Add in the egg and vanilla extract, scraping down the sides as necessary, and mix until well combined and smooth; around 1 minute on medium-high speed.

Add in the baking powder, soda, salt and flour and mix on low until the dough starts to pull away from the sides and comes together.

Tip into your prepared pan and press into a level layer. I used the bottom of a measuring cup to ensure a smooth, level layer.

Place in the oven for 10-12 minutes until risen, lightly golden and puffy.

Whilst the base is baking; place oats, sugar, flour and butter into a medium sized bowl and rub together with your fingers until you have a coarse breadcrumb texture. Alternatively you can use a pastry blender to cut the butter into the flour. Leave to one side.

Once the base is risen and golden, remove from the oven and then sprinkle the fresh blueberries all over the cookie dough. Lightly press with your hands, so that a few of the blueberries sink into the dough; make sure to leave plenty sitting on the top too!

Sprinkle the crumble topping all over the blueberries, it's fine to have some blueberries peeking through the top of the crumble. Press down lightly on the crumb topping to help it stick to the dough.

Place in the oven for 25-30 minutes until the blueberries are bubbling and the top is evenly golden. An inserted skewer into the centre should not have cookie dough on it when removed.

Leave to cool completely in the pan, before transferring to a cutting board to slice into 9-12 slices.

Blueberry Sugar Cookie Bars will keep in an airtight container, at room temperature for 3 days.