In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, beat the butter, sugar and salt on medium speed for 4-5 minutes or until light and fluffy. Beat in egg until combined. Scrape down the sides and bottom of the bowl as needed.
Sift the flour and cocoa powder together in a medium bowl. On low speed, slowly add flour mixture to butter mixture. Do not overmix but make sure all the flour is incorporated.
Turn the dough onto a surface and push together into a flat square. Wrap in plastic wrap and place in fridge for 1 hour.
Preheat oven to 350°F. Remove dough and divide into 4 equal pieces. Place the dough between two pieces of parchment paper and roll to ⅛" thickness. Use a small circle cookie cutter and cut into individual rounds and place on a parchment-lined baking sheet. Leave at least ½" between each cookie.
Bake for 15 minutes, rotating the pan halfway through. Remove from oven and transfer to cooling rack.
While cookies are cooling, make cream filling. In a large bowl using a hand-held mixer or a stand mixer fitted with a whisk attachment, beat the butter, sugar and vanilla extract until light and fluffy, about 2-3 minutes.
Assemble the cookies by spreading ½ tablespoon of filling between two cookies and press. Scrape away the excess filling to even out the sides.
Notes
Dark cocoa powder is the key to Oreos. You don't get that dark color the Oreo is known for with regular cocoa powder. Hershey's Special Dark Cocoa is my go-to.
Roll the dough between two pieces of parchment paper so it doesn't stick.
If you want slightly softer cookie, bake for 12-13 minutes or slightly crunchier, bake for 17-18 minutes. For me, 15 minutes created the right consistency.
These homemade Oreo cookies will last about two weeks in an airtight container at room temperature. You can also freeze for up to three months.