Beat the granulated sugar, cocoa powder, and vegetable oil in a large bowl with a hand mixer or paddle-equipped stand mixer on medium speed. Stop when the mixture looks evenly blended, dark, and shiny with no dry cocoa pockets.

Beat in the eggs one at a time, mixing well after each addition, then blend in the vanilla extract. The mixture should become glossy and fluid-looking before the dry ingredients are added.

Stir the flour, baking powder, and salt together in a separate medium bowl, then mix them into the cocoa mixture on low speed just until no flour streaks remain. The finished dough will be thick and sticky, which is exactly what creates a fudgy center after chilling.

Cover the dough with plastic wrap and refrigerate for at least 4 hours or up to overnight. After chilling, it should feel firm enough to scoop and roll without flowing across your hands.

Heat the oven to 350°F, line two large baking sheets with parchment paper, and place the powdered sugar in a small bowl. Let the oven fully preheat so the chilled dough gets the quick heat it needs to rise and crack.

Scoop the chilled dough with a small ice cream scoop or tablespoon, roll it into roughly one-inch balls, coat each ball well in powdered sugar, and arrange them about 2 inches apart on the prepared sheets. Coat only the dough balls that fit on the current sheets, keeping the rest chilled until it is time to bake them.

Bake for 10 to 12 minutes, then leave the cookies on the sheets for a few minutes before moving them to a wire rack to cool completely. Pull them when the edges are firm and the puffed centers still look slightly wet through the cracks.