Preheat oven to 350°F and spray a 13x9-inch baking pan with nonstick spray. Set aside.
In a small bowl, whisk the flour and salt together; set aside.
1 cup all-purpose flour, ½ teaspoon kosher salt
In a large bowl, using a hand mixer, beat the butter and sugar together until lighter in color and fluffy, about 2 minutes.
½ cup salted butter, 1 cup granulated sugar
Add eggs, one-at-a-time. Beat well after each addition.
4 large eggs
Add syrup and vanilla. Beat well.
16 ounces chocolate syrup, 1 teaspoon pure vanilla extract
Sprinkle the flour/salt mixture over the chocolate mixture and mix just until no dry flour remains.
Pour into the prepared baking pan.
Bake for 30 minutes (the brownie top will still appear wet). Transfer brownies to a wire rack to cool completely.