For the Chocolate Batter: In a microwave-safe bowl, melt butter and chocolate for about 1 minute. Remove and stir well.
In a bowl, add eggs, sweetener, and vanilla and blend until light and frothy.
Pour the melted butter and chocolate into the bowl in a slow stream and beat again until it is well-incorporated.
Pour the mixture into the pan.
For the Cheesecake Batter: beat together cream cheese, egg, Truvia and vanilla extract. Pour the cheesecake batter on top of the brownie batter. Use a spatula to swirl the two batters together.
Cover the pan with foil. Bake at 350F for 45-50 minutes until a knife inserted in the center emerges clean.