Butter the cake pan and line the bottom with a parchment round. Butter the top of the parchment and set aside.
Melt the chocolate in the microwave by heating in 20-second increments, stirring in between until fully melted. Add the softened butter and mix to combine. The mixture should be creamy and the butter should not be melted.
Combine the sugar, salt, and cornstarch, whisk into the chocolate mixture.
Add the room temperature eggs and vanilla extract, then whisk again.
Finish by adding the COCOA POWDER, mixing with a spatula until smooth.
Bake at 375°F (191°C) for 25-30 minutes, or until a cake tester comes out clean. Remove from oven and set aside to cool. While still warm, invert onto a serving dish and continue to cool.