Preheat the oven to 350°F. Line an 8-by-8-inch square baking pan with parchment paper or grease it well with cooking spray; a springform pan may also be used.

In a large bowl, whisk the melted butter, granulated sugar, and cocoa powder vigorously for about 1 minute, until well combined and smooth.

Whisk in the eggs one at a time, followed by 1 teaspoon vanilla extract. Keep whisking for another minute, until the batter is smooth and shiny.

Using a spatula, stir in the flour and salt just until the flour disappears. Do not overmix.

Gently fold in the chocolate chips or chopped walnuts, if using.

Spread the batter evenly into the prepared pan.

Bake for 20 to 22 minutes, until the center is slightly set and a toothpick inserted into the middle comes out with a few moist crumbs, not wet batter.

Cool the brownie.

For the no-bake cheesecake, place the softened cream cheese, powdered sugar, sour cream or Greek yogurt, and 1 1/2 teaspoons vanilla extract in a large bowl. Beat with an electric mixer on medium-high speed until entirely smooth and lump-free.

Gently fold the thawed whipped topping or stiff homemade whipped cream into the cream cheese mixture using a spatula.

Pour the cheesecake mixture over the cooled brownie and spread it evenly.

Refrigerate until set.

Remove from the refrigerator, sprinkle cinnamon sugar on top, and serve.