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 chips. Fold through.
Chill the dough in the fridge for 30 minutes.
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.
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 Thermomix bowl. Sift by pressing Turbo 5-10 times. Set aside in a separate bowl.
Cream the softened butter, brown sugar and caster sugar for 30 seconds, Speed 4. 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 measuring cup hole. Add the extra egg yolk and continue to mix on Speed 2.
Add the vanilla extract and the flour mixture and mix for 30 seconds on Speed 4, (using the spatula to assist).
When the dough is completely combined, add the rolled oats and mix for 30 seconds on Reverse, Speed 3 (using the spatula to help mix).
Fold through the chocolate bits.
Chill the dough in the fridge for 30 minutes.
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.
Notes
Chilling the dough - chilling the dough before baking allows the fat to re-solidify. When you bake chilled cookie dough, it will spread more slowly meaning that the outside will cook quicker while the inside will cook slower (and stay chewy!)
Tray space - even when the dough has been chilled, these cookies will spread a LOT so it’s important to leave a large (5-7cm) gap between them on the baking trays.
Storing - these cookies can be stored in an airtight container at room temperature for 1 week.
Freezing - you can freeze the unbaked dough in a sealable plastic bag for up to 3 months. Alternatively, you can freeze the baked cookies in an airtight container for 3 months (note: the cookies will lose their crispiness when defrosted).