Preheat the oven to 325°F. Line 2 baking sheets with parchment paper.

In the bowl of a stand mixer, or with a heavy-duty hand mixer, mix together 3/4 cup granulated sugar, melted chocolate, oil, oat milk, corn syrup, flaxseed, and vanilla extract.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually mix in just until a thick dough comes together.

Refrigerate the dough for 30 minutes, until chilled throughout. Meanwhile, pour the remaining 1/4 cup granulated sugar into a small bowl and set it aside. Sift the confectioners' sugar into a separate small bowl and set it aside.

For each cookie, scoop a heaping tablespoon of dough and roll it into a ball. Roll the ball first in the granulated sugar then roll in the confectioners' sugar. Set on a cookie sheet at least 2 inches apart.

Bake 14 minutes, until cookies are puffed and cracks are visible. Let cookies stand on the baking sheet for 5 minutes then transfer to wire racks until cooled completely.