Make the Frosting
In a small bowl, stir the kuzu root starch into the coconut milk until dissolved. In a medium saucepan, whisk the agar into the water and bring to a boil. Simmer over moderately low heat, whisking occasionally, until very thick, 4 minutes. Whisk in the maple syrup, cocoa powder and salt until fully combined. Return to a boil, whisking constantly. Add the kuzu-coconut milk mixture and whisk until smooth. Simmer over moderately low heat, stirring constantly, until very thick and pudding-like, 5 minutes. Remove from the heat and whisk in the chocolate chips, coconut oil and vanilla until the frosting is smooth and glossy. Pour into a glass bowl and let cool to room temperature. Cover with plastic wrap and refrigerate until firm, at least 2 hours or overnight.