After several attempts at various recipes, I finally found Sweetopia's and won't use anything else.
Do try to use the highest quality cocoa powder you can, even if that means ordering some online. You really can tell the difference in both color and taste. The cookies pictured, for example, were made with Callebaut Baking Cocoa Powder and ended up tasting like brownies (soooo good!).
Be careful to not overbake the cookies. I always error on the side of underbaking these (as strange as that might sound) because the cookies will keep baking when they're removed from the oven. I've baked too long only to have the cookies harden so much they were like bricks once they cooled. Test your oven by starting with 8 minutes, cool, taste and increase the baking time from there.
DARK CHOCOLATE SUGAR COOKIES
2 cups unsalted butter (at room temperature)
2 cups sugar
2 large eggs
3 teaspoons vanilla
4 cups all-purpose flour
1 1/2 cups cocoa powder
1 teaspoon salt
In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugars together on low to medium speed until thoroughly incorporated (about one minute). Scrape down the sides of the bowl with a plastic spatula and mix again for a few seconds more.
Add eggs slowly and mix until combined. Scrape down the bowl with your spatula at least once and mix again.
Add vanilla extract. Mix until combined.
In separate bowl, sift or whisk your dry ingredients together.
Add all the flour mixture to the mixing bowl. Mix on low speed (stir) for 3o seconds to reduce spillage. Increase to low speed and observe the dough mixing; when it clumps around the paddle attachment it’s ready.
Roll the dough out between 2 large pieces of parchment paper. Place on a baking sheet and into the fridge for a minimum of 1 hour.
Roll out the dough further if you need to and cut out cookie shapes. Place on parchment paper-lined baking sheet. Re-roll scraps and repeat.
Put cookie dough cutouts back in the refrigerator for a minimum of 10 minutes to chill again. This helps them hold their shape better when baking.
Preheat your oven to 350°F.
Bake cookies for 8-12 minutes or until the center of the cookie no longer looks wet.
Let cookies cool for 10 minutes and then move to wire cooling rack until they're at room temperature.
Decorate! Or not! These cookies are great on their own, but also hold royal icing wonderfully.