Preheat oven to 350°F. Line a 9×9 pan with parchment paper and lightly spray with baking spray.
Press the chocolate chip cookie dough evenly into the bottom of the pan to create the base layer.
Bake for 10–12 minutes, just until the dough is set around the edges but still soft in the center.
While it bakes, divide the sweetened condensed milk into two bowls. Tint one blue and the other purple.
Remove the pan from the oven and drizzle both colors over the cookie base, gently swirling to create a galaxy effect. Sprinkle the white chocolate chips evenly over the top.
Arrange the Yoshi egg cookies in a single layer on top. Let them sit on the surface without pressing them down.
Return to the oven and bake for another 12–14 minutes, until the base is golden and the top cookies are baked through.
While still warm, finish with star sprinkles or edible glitter.