Preheat your oven to 350°F (175°C).

Line an 8-inch by 8-inch baking dish with parchment paper and coat it with cooking spray to prevent sticking.

In a large mixing bowl, combine ½ cup of softened butter, ¾ cup of Splenda, and 1 tsp of vanilla extract. Beat until creamy and well-mixed.

Add in the three eggs and three egg whites, mixing until fully incorporated.

Gradually add ¾ cup of King Arthur Keto Wheat Flour, ⅔ cup of dark cocoa powder, 1 tsp of baking powder, and ½ tsp of salt. Mix until just combined; the batter will be thick and glossy.

Pour the brownie batter into the prepared dish, smoothing the top with a spatula.

In a separate bowl, beat the 6 ounces of softened low-fat cream cheese until smooth. Add the remaining 1 large egg, ½ tsp of vanilla extract, and ¼ cup of Splenda, mixing until creamy.

Spoon the cream cheese mixture over the brownie batter. Use a knife or spatula to create swirls for a marbled effect.

Bake in the preheated oven for 30-35 minutes, or until the edges are set and a toothpick inserted into the center comes out mostly clean.

Allow cooling for at least 15 minutes before slicing into squares.