For the brownies:
Place the butter and chocolate into a medium saucepan and melt gently over low heat while stirring frequently. Remove the mixture from the heat and allow to cool. Preheat the oven to 350°F. Line a 13Χ9 inch baking pan with aluminum foil. Coat the foil lightly with non-stick cooking spray.
Using an electric mixer, beat the sugar and eggs together in a medium bowl until light and fluffy (this took about 3-4 minutes with my mixer). Sift the flour and salt together; add to the sugar mixture in three half-cup increments. Add in the chocolate mixture by folding it into the sugar mixture.
For the cream cheese filling:
In another medium bowl, combine the cream cheese and sugar and beat until creamy. Continue to beat while adding the egg and liqueur.
To assemble the brownies:
Pour half of the brownie batter into the pan. Spread all of the the cream cheese layer over the brownie layer. Pour the remaining brownie batter over the cream cheese layer and using an offset spatula, smooth the batter. To create a marbled effect, swirl the layers together with a knife. Bake for 40 minutes; cool completely before cutting.