Add butter and peanut butter to a microwave-safe bowl. Melt in the microwave at 50% power in 30 second intervals, stirring after each interval. Once melted, cool to room temperature.
Add cooled butter/peanut butter to a large mixing bowl. Add sugar and brown sugar and whisk until well combined.
Add milk and ground flax/chia and mix.
Add oats and sift in cocoa powder, baking soda, and baking powder. Mix with a spoon or spatula until well combined.
Cover and place the cookie dough in the fridge for 30 minutes.
Preheat oven to 350°F/180°C. Line a cookie sheet with parchment paper.
Scoop about 2 tbsp of dough and roll into a ball. Place on cookie sheet and lightly flatten into a disc (the cookies don't spread much on their own). Repeat with remaining dough, leaving at least an inch of space between each cookie.
Bake for 9 minutes. Cool on the cookie sheet for 2 minutes, then transfer cookies to a wire rack to finish cooling.