Preheat oven to 350 degrees F. Spray a 9x9 inch square baking pan with nonstick spray, line with parchment paper leaving an overhang of 1-inch on the sides, then spray parchment paper.
Prepare fudge brownie batter according to package instructions and bake for 20 minutes. In a medium bowl, whisk to combine flour, baking powder and salt. In a separate bowl or measuring cup, whisk to combine milk and sour cream.
To make the butter cake batter, in the bowl of a stand mixer with paddle attachment, beat to combine butter and granulated sugar on medium speed until light and fluffy, about 1-2 minutes. Add 2 eggs, one at a time, and beat until combined. Reduce speed to low and add flour and milk in 3 additions, beginning and ending with the flour; set batter aside.
To make gooey topping, beat the cream cheese until smooth, then beat in 2 eggs, confectioner’s sugar and lemon juice until smooth and well combined.
Spoon the butter cake batter onto the brownie and use a small offset spatula to spread evenly over the brownies. Pour gooey topping over the batter and smooth into an even layer. Bake for 45-50 minutes until topping baked through but still soft when tested with a cake tester or toothpick.
Let cool, then dust top with confectioner’s sugar. Cut into squares to serve.