Preheat the oven to 350°F (180°C). Line an 9-inch square baking dish with parchment paper.

In a large microwave safe bowl, combine the melted butter and sugar. Microwave the mixture for 1 minute. Whisk until the mixture gets to a glossy finish.

Beat in the eggs one at a time, whisking in between. Whisk until the mixture becomes fluffy and light in color, for about 2 minutes.

Add in cocoa powder, cornstarch, salt, and vanilla extract. Fold in flour until just combined.

Pour the batter into prepared baking pan and bake for about 20 minutes.