Preheat the oven to 350 ℉. Line a baking sheet with parchment paper or a silicone baking mat.
In mixing bowl, stir flour, oats, baking powder, baking soda, and salt. Set aside.
Using a standing mixer or a hand-held mixer with a large bowl, beat the butter on medium to medium high, about 2-3 minutes. You want it to be a pale yellow and very creamy.
Add sugar and beat another 2-3 minutes until light and fluffy, scraping down the sides of the bowl with a rubber spatula as needed.
Add the molasses and mix until fully incorporated, again scraping down the sides of the bowl as needed.
Add eggs, one at a time, allowing each egg to incorporate before adding the next.
Slowly add the vanilla extract and milk and mix until incorporated. At this point the dough might look a bit watery and curded. That’s totally fine it will firm up once you add the flour mixture.
Add the flour/oat mixture and mix on low until just incorporated. Scrape down the sides of the bowl with a spatula to make sure the dough is fully mixed and all the flour and oats have been incorporated.
Turn off the mixer a gently fold in the chocolate chips and walnuts using a spatula.
Using an ice cream scoop or spoon, add balls of 2-3 tablespoons of dough to prepared baking sheet, leaving around 2 inches between each cookie.
Use the bottom of cup to flatten each ball of dough to about a 2 ½ to 3 inch circle. Putting a piece of parchment or wax paper between the dough and the cup will prevent the dough sticking to the cup.
Place the baking tray in the oven and bake 10-12 minutes until the edges are golden brown.
When the cookies are done, remove from oven, let cool, and enjoy!