Preheat oven to 160 degrees (fan-forced). Grease and line 3-4 baking trays with baking paper.
Cream the softened butter, brown sugar and caster sugar with handheld beaters or a stand mixer until pale and creamy.
Add the egg and continue to beat. Add the extra egg yolk and continue to beat.
Add the vanilla extract and beat until well combined.
Sift in the plain flour, bi-carb soda and baking powder, then add the rolled oats and chocolate chunks. Fold through.
Roll the mixture into heaped tablespoon-sized balls and place 3-4cm apart on the prepared trays. Press down lightly.
Bake for 12-15 minutes or until lightly golden around the edges.
Sprinkle sea salt flakes over the still-warm cookies (optional).
Allow to cool on the baking trays for 10 minutes before moving to a wire rack to cool completely.
Store in an airtight container for up to 5 days.
Thermomix Method
Preheat oven to 160 degrees (fan-forced). Grease and line 3-4 baking trays with baking paper.
Place the plain flour, baking powder and bi-carb soda into the TM bowl. Sift by pressing Turbo 5-10 times. Set aside in a separate bowl.
Cream the softened butter, brown sugar and caster sugar on Speed 4, 30 seconds. Scrape down the sides of the bowl and repeat until pale and creamy (approximately 1 more minute).
Reduce to Speed 2 and add the egg through the MC hole. Add the extra egg yolk and continue to mix on Speed 2.
Add the vanilla extract and the flour mixture and mix on Speed 4, 30 seconds (using the spatula to assist).
When the dough is completely combined, add the rolled oats and mix on Reverse, Speed 3, 30 seconds (using the spatula to help mix).
Fold through the chocolate chunks. Roll the mixture into heaped tablespoon-sized balls and place 3-4cm apart on the prepared trays. Press down lightly.
Bake for 12-15 minutes or until lightly golden around the edges.
Sprinkle sea salt flakes over the still-warm cookies (optional).
Allow to cool on the baking trays for 10 minutes before moving to a wire rack to cool completely. Store in an airtight container for up to 5 days.
Notes
The cookie dough can be made ahead of time and frozen for up to 3 months.