Transfer the soft butter to a large bowl, add sugar and vanilla - beat together with a mixer on high speed for a few minutes.

Add one egg at a time to the butter and mix briefly after each addition - just until combined.

Sift flour, baking soda, baking powder, and salt through a fine sieve into the butter mixture.

Mix briefly with a mixer on low speed - just until combined.

Add oat flakes to the cookie dough.

Add the add-ins - for me, it's chopped walnuts and coconut flakes, and chopped white chocolate.

Mix everything with a spoon - the mixture will be thick.

Scoop large portions of dough with a spoon - my daughter and I rolled them into balls.

Arrange the cookies on a large baking sheet lined with parchment paper - leave space between the cookies.

Lightly flatten each cookie with the palm of your hand.

Place the baking sheet with cookies into a preheated oven at 170 degrees (fan-assisted) and bake for about 16-18 minutes - until nicely browned.

Remove the baking sheet from the oven and let cool for about 10 minutes - during this time, the cookies will slightly harden and can be removed from the baking sheet (they are still soft and delicate at first).