Follow these steps carefully for perfectly fudgy and beautifully crinkled cookies:

1. Combine Wet Ingredients & Sugar: In a large mixing bowl, combine the granulated sugar and vegetable oil. Whisk them together until well combined.
2. Add Cocoa Powder: Add the unsweetened cocoa powder to the sugar and oil mixture. Whisk thoroughly until you have a smooth, glossy, and very thick chocolate paste. This step is crucial for blooming the cocoa and developing a deep chocolate flavor.
3. Incorporate Eggs & Vanilla: Beat in the eggs one at a time, ensuring each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed. Once all eggs are in, stir in the vanilla extract. The mixture will become thinner and very glossy.
4. Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, and salt. This ensures the leavening agent and salt are evenly distributed.
5. Combine Wet and Dry: Gradually add the dry ingredient mixture to the wet chocolate mixture. Mix on low speed with an electric mixer, or by hand with a sturdy spatula, until just combined. Be careful not to overmix; overmixing can lead to tough cookies. The dough will be very soft, sticky, and resemble thick brownie batter.
6. Chill the Dough (Crucial Step!): Cover the bowl tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight (up to 24 hours). This chilling step is essential. It allows the dough to firm up, making it manageable to roll, and it helps the cookies hold their shape and crinkle properly during baking.
7. Preheat Oven & Prepare Baking Sheets: When ready to bake, preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats.
8. Roll in Powdered Sugar: Place the powdered sugar in a shallow bowl or pie dish. Scoop out tablespoon-sized portions of the chilled dough (a small cookie scoop, about 1 to 1.5 tablespoons, works perfectly). Roll each portion between your palms to form a smooth ball.
9. Coat Generously: Drop each dough ball into the powdered sugar and roll it around to coat generously and completely. Don’t skimp on the sugar; a thick coating is key to a prominent crinkle.
10. Arrange on Baking Sheets: Place the sugar-coated dough balls about 2 inches apart on the prepared baking sheets. They will spread during baking.
11. Bake: Bake for 10-12 minutes. The cookies should be puffed up, and the tops should have distinct crinkles. The edges will be set, but the centers will still look slightly soft and fudgy. Be careful not to overbake, as this will result in dry cookies rather than fudgy ones.
12. Cool: Let the cookies cool on the baking sheets for 5-10 minutes. They will be quite soft when they first come out of the oven and will firm up as they cool. After 5-10 minutes, carefully transfer them to a wire rack to cool completely. The crinkles will become more defined as they cool.