In a stand mixer with the paddle attachment, cream the butter and sugar on high speed until light and fluffy. Add the sweetened condensed milk and vanilla and mix on high speed for 5 minutes. Scrape down the sides of the bowl.
In a separate bowl, whisk/sift together the flour, baking powder, espresso powder, cocoa powder and salt.
With the mixer on low speed, add the flour mixture in 3 batches. Turn the mixer up to medium for a minute to make sure everything is evenly combined.
Preheat the oven to 350 degrees F. Line 2 large baking sheets with parchment paper.
Use a medium or small scoop to scoop out the dough. These cookies will spread quite a bit so leave space in between them.
Allow the dough to chill in the refrigerator for at least 25 minutes.
Bake for 15-18 minutes. Allow to cool for several minutes before serving.
Tip: this cookie dough is a bit thinner and stickier than normal. To keep the shape of the cookies pretty while scooping, try washing and drying the scoop periodically with hot water. Make sure to thoroughly chill the scooped dough before baking.