Sift together the cornstarch and confectioners' sugar, then stir in the finely crushed Oreo crumbs. Line a 9x13 metal baking pan with nonstick spray, then dust generously with one third of the coating mixture. Set aside. Coarsely chop the Oreos for folding in and set aside.
¼ cup powdered sugar, ¼ cup cornstarch, ½ cup Oreo cookies, non stick cooking spray, 1 cup Oreo cookies
In a stand mixer bowl, pour in ½ cup cold water and sprinkle the gelatin evenly over the surface. Let bloom for 5 minutes.
2 ½ tbsp unflavored gelatin, 1 cup cold water
Combine ½ cup water, granulated sugar, corn syrup, and salt in a saucepan. Cook covered over medium high heat until simmering, about 3 minutes. Uncover and cook without stirring until it reaches 240°F, about 6 to 7 more minutes.
1 ½ cups white sugar, 1 cup light corn syrup, ¼ tsp salt, 1 cup cold water
With the mixer on low, slowly pour the hot syrup into the bloomed gelatin. Increase to high speed and whip until thick and lukewarm, 13 to 14 minutes. Add the vanilla and whip for 30 seconds more.
1 tsp vanilla
Turn off the mixer. Using a spatula, gently fold the coarsely chopped Oreos through the marshmallow by hand until evenly distributed.
1 cup Oreo cookies
Pour into the prepared pan and smooth the top. Dust with more coating mixture. Let sit uncovered for at least 3 hours, or overnight.
Turn out onto a cutting board, dust with coating mixture, and cut into squares with an oiled knife. Coat all sides of each piece before serving or storing.
Notes
Fold the cookie chunks in gently by hand once the marshmallow is fully whipped, a heavy hand will knock out the air.
Chop the fold-in pieces with a knife rather than a food processor so you get real chunks, not crumbs.
Store in an airtight container at room temperature.