In a medium bowl, whisk together the flour, cocoa powder, cornstarch, baking soda and salt. Set aside.
In a large bowl, beat the dairy free butter, brown sugar and granulated sugar for 2 minutes until fluffy. Beat in the egg, oat milk and vanilla until smooth.
Add the dry ingredients and mix on low speed until a thick dough forms, then fold in the chocolate chips. Cover and chill for 30 minutes.
Preheat the oven to 350°F and line two baking sheets with parchment paper. Use kitchen scissors to snip each marshmallow in half, then roll the dough into 2 tablespoon balls and place them 3 inches apart.
Bake for 8 minutes, then remove the pan and gently press a marshmallow half, cut side down, onto the center of each cookie. Return to the oven for 2-3 minutes, just until the marshmallows puff and soften.Let the cookies cool on the sheet for 10 minutes, then transfer them to a wire rack.
For the drizzle, microwave the chocolate chips with the coconut oil in 30-second bursts, stirring between each, until smooth. Drizzle over the cooled cookies and let the chocolate set for 20 minutes.