Preheat oven to 350 degrees Fahrenheit.

In a large bowl, microwave the semisweet chocolate and butter on high for 2 minutes, or until the butter is melted.

Stir until the chocolate is completely melted and smooth.

Add 3/4 cup of sugar to the chocolate mixture and mix well.

Blend in the two beaten eggs and vanilla extract.

Stir in 1/2 cup of flour and chopped nuts until well combined.

Spread the brownie batter evenly into a greased 8-inch square baking pan.

In a small bowl, mix together the cream cheese, 1/4 cup of sugar, one egg, and 1 tablespoon of flour until well blended and smooth.

Spoon the cream cheese mixture over the brownie batter in dollops.

Use a knife to cut through the batter several times to create a marbled effect.

Bake in the preheated oven for 35 to 40 minutes, or until a wooden pick inserted into the center comes out almost clean. Avoid overbaking.

Allow the brownies to cool completely before cutting them into squares and serving.