Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Melt and brown the butter: place the butter in a skillet over medium heat and cook, stirring occasionally, until it smells nutty and small brown bits form, about 3–4 minutes. Remove from heat and transfer the butter to a heatproof bowl; let cool for about 5 minutes.
Stir the brown butter with the brown sugar, then add the eggs and vanilla and mix until smooth and combined.
Add the flour, baking soda, baking powder, and kosher salt to the bowl and fold until just combined.
Fold in the chocolate chunks or chips, old-fashioned oats, coconut flakes, and roasted peanuts.
Drizzle 1/2 cup sweetened condensed milk over the dough and gently mix until the dough is lightly streaked with condensed milk (it should not be fully incorporated).
Scoop the dough into 1/4-cup portions (about 20 cookies), place them 3 inches apart on the prepared baking sheet, and flatten each ball slightly. Drizzle about 1 teaspoon condensed milk over each cookie.
Bake for 8 minutes, rotate the pan, then bake an additional 3–5 minutes until edges are set and centers are still slightly soft.
Remove from the oven, optionally press 3–4 chocolate chips onto each warm cookie, and let the cookies cool on the baking sheet; they will continue to set as they cool.
Serve warm or let cool completely before storing.