Preheat the oven to 350°F (175°C). Line an 8-by-8-inch baking pan with aluminum foil and spray the foil lightly with cooking spray; set the pan aside.
In a large, microwave-safe bowl, melt ½ cup (1 stick) unsalted butter in the microwave for about 1 minute on high (or until fully melted). Let the butter sit and cool for about 1–2 minutes so it is warm but not hot.
Add 1 large egg to the cooled melted butter and whisk until combined. Then add 1 cup packed light brown sugar, 1 tablespoon vanilla extract, and 2 to 3 teaspoons cinnamon (use 3 teaspoons for stronger cinnamon flavor; reduce if sensitive). Whisk until the mixture is smooth.
Add 1 cup all-purpose flour and ½ teaspoon salt to the wet mixture. Stir with a spatula until just combined and no large streaks of flour remain; do not overmix.
Pour the batter into the prepared pan and smooth the top lightly with a spatula.
Bake for 20 to 24 minutes, or until the top is set in the center and a toothpick inserted in the center comes out clean or with a few moist crumbs (not wet batter). Remove from the oven and place the pan on a wire rack.
Cool the blondies in the pan on the wire rack for at least 90 minutes (they will firm up as they cool) before frosting.
While the blondies cool (or after they are cool), make the frosting: in a large bowl or the bowl of a stand mixer, beat 6 ounces softened cream cheese and ¼ cup softened unsalted butter until smooth.
Add 1½ cups confectioners’ sugar, ½ teaspoon vanilla extract, and ½ teaspoon salt to the cream cheese mixture. Beat with an electric mixer (or paddle attachment) until the frosting is light and fluffy, about 2 to 3 minutes.
Spread the frosting evenly over the completely cooled blondies using a spatula or knife. Sprinkle a light dusting of cinnamon over the top for garnish.
Slice into bars and serve. Store assembled bars airtight at room temperature for up to 4 days or in the refrigerator for up to 1 week. If refrigerated, you may want to let bars come to room temperature before serving for a softer texture.