Preheat oven to 170 degrees celsius (160 degrees if fan forced). Grease and line a 17cm x 28cm slice tray with baking paper, allowing the edges to overhang.
Melt the butter and vanilla in the Thermomix bowl for 3 minutes, 50 degrees, Speed 2.
Add the plain flour, rolled oats, salt, bi-carb soda, brown sugar and raw sugar. Mix for 20 seconds on Reverse, Speed 2. Scrape down the sides. Mix for a further 20 seconds on Reverse, Speed 2.
Spread half of the crumb mixture into the bottom of the prepared tin and press into an even layer. Bake in preheated oven for 15 minutes. Remove from oven and set aside (leave the oven turned on).
Peel the zest (yellow part only) off a lemon using a grater or knife. Place the zest into the Thermomix bowl and grate for 20 seconds, Speed 6. Scrape the sides of the bowl. Repeat until you have a very finely grated zest.
Add the condensed milk, lemon juice, egg yolks and lemon extract and mix for 30 seconds, Speed 4. Scrape the sides of the bowl and mix for a further 30 seconds, Speed 4.
Pour mixture over crumb crust in baking dish and spread in an even layer. Sprinkle the remaining crumble mixture evenly over the top of the lemon layer.
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 cut into squares. Store in an airtight container in the refrigerator.