In a medium saucepan, melt the butter over medium heat. Avoid using a dark pan, as you will struggle to see the caramelization of the butter and run the risk of burning it.
Once the butter melts, let it cook for around 3-4 minutes, regularly whisking.
Once it starts to foam, add in milk powder and whisk until it browns into a deep amber color and smells nutty and caramel-like.
Pour the melted butter, including all the caramelized brown bits at the bottom of the pan, into a large heat-proof bowl and allow it to cool for 10 minutes.
Preheat the oven to 350°F/180°C and line large baking sheets with parchment paper.
Once the butter has cooled, whisk in the brown sugar, granulated sugar, egg, and vanilla.
In a small separate bowl, whisk the flour, rolled oats, salt, and baking soda. Stir the dry ingredients into the wet ingredients, followed by the raisins.
Let the dough sit for 10 minutes at room temperature.
Roll a tablespoon and a half of dough per cookie into a ball. Place the cookie dough balls around 2 inches/ 5 cm apart on the prepared baking sheets.
Bake the cookies for 10-12 minutes until the cookies are golden brown around the edges but still a little soft. Use a large round cookie cutter to scoot around the baked cookies to push them into even circles.
Let the cookies cool for 5 minutes on the baking sheet before placing them on a wire rack to cool further.
These cookies can be stored in an airtight container at room temperature for up to a week, or the baked cookies can be frozen for up to three months.
Notes
*The cup sizes given are for US cups. Note that these are smaller than metric cup sizes. For best results, use grams.