Preheat the oven to 350°F and line two baking sheets with parchment paper.
Place the butter and chocolate chips in a microwave safe bowl, and microwave for about 1 minute. Remove and stir until melted. Set aside.
In the bowl of a stand mixer with a whisk attachment or in a large bowl using a hand mixer, beat the eggs and sugar for about 3-4 minutes on high speed until fluffy and the eggs are paler in color.
Pour in the chocolate mixture and vanilla extract and mix. Then add the Eagle Brand condensed milk and mix until smooth. Add the flour, cocoa powder, baking powder and salt. Fold in the dry ingredients until combined.
Add about 1 cup of the marshmallow fluff to the bowl. Gently fold it in just 1-2 times. You don’t want it fully mixed in so that the marshmallow will show on top of the cookie dough when baking.
Using a 2-3 tbsp cookie scoop, drop 6-9 cookies on each prepared pan. Use additional marshmallow fluff to add more to the top of each cookie, and swirl with a knife or toothpick, if desired to see the brown and white marbling. Make sure to leave a little space between each cookie because they will spread when they bake.
Bake one tray at a time for 10-12 minutes. Remove the tray from the oven and immediately place a cookie cutter, larger than the cookie, and use the cookie cutter to mold the edges of the cookie from uneven to perfectly round, using a swirling motion.
Allow the cookies to cool for at least 15 minutes before removing them from the parchment paper.
Repeat the baking process with the remaining batter, using a new sheet of parchment paper each time.