Combine the agar flakes and water over medium heat for 3 minutes, whisking occasionally. Then whisk in the cocoa powder, maple syrup, vanilla and salt.
Combine the cornstarch and almond milk in a small bowl and add the cornstarch slurry to the agar mixture. Continue to cook, stirring constantly, until the mixture starts to bubble and thicken. Let cook for 1 full minute to activate the cornstarch, then remove from heat.
Pour into a glass pan or measuring cup and refrigerate for at least 20 minutes, or until the cake is ready to be frosted. Whisk the frosting by hand or with an electric beater to fluff up before frosting the cake.