Preheat oven to 160 degrees celsius (fan-forced.
Grease and line a 18cm x 28cm slice tray with baking paper, allowing the edges to overhang.
In a large bowl, whisk together the plain flour, rolled oats, salt and bi-carbonate of soda.
Stir in the raw sugar and brown sugar and mix until smooth.
Stir vanilla extract into melted butter and pour mixture over dry ingredients.
Stir mixture until well combined.
Press just over half of the crumble mixture into the base of the prepared tin and use the back of a spoon to smooth the mixture into an even layer. Bake in the preheated oven for 15 minutes, until golden.
Remove from oven and put aside (leave the oven turned on).
Sprinkle the blueberries over the cooked crumble base.
In a bowl, whisk together sweetened condensed milk, lemon zest, lemon juice, egg yolks and lemon extract (optional) until smooth and slightly thickened.
Pour the lemon mixture over the blueberries and spread into an even layer.
Sprinkle the remaining crumble mixture over the top of the lemon filling and blueberries.
Bake in oven for 23 - 26 minutes or until lightly golden.
Remove from oven and allow to cool.
Cover and refrigerate for at least 2 hours (preferably overnight) then take it out and cut into squares. Store in airtight container in the refrigerator for up to 1 week.