Place the butter and chocolate in a medium saucepan and melt over low heat, stirring often. Remove from the heat and let cool.
Preheat oven to 350°F (175°C). Line a 13" × 9" x 2" baking pan with parchment paper or aluminum foil and lightly coat with a non-stick cooking spray.
Beat the sugar and eggs together in a medium bowl with an electric mixer until lightened and fluffy (approximately 4 minutes).
Add the flour and salt gradually, mixing just until combined.
Fold in the melted chocolate mixture.
To make the filling:
Using an electric mixer on medium speed, beat the cream cheese and sugar until creamy.
Add the egg and liqueur, and beat until smooth.
To assemble:
Pour half of the brownie batter into the pan. Spread all of the cream cheese filling over the chocolate layer.
Pour the remaining brownie batter over the cream cheese layer and smooth evenly. Leave the layers as they are, or swirl the top two layers together with a knife for a marbled effect.
Bake for 40 to 45 minutes, or until a pick inserted into the center comes out with moist crumbs.
Place the pan on a wire rack, and allow the brownies to cool completely before cutting into bars.
Notes
Recipe slightly adapted from a clipped recipe in my collection. I’m afraid I don’t know the original source.
Store in an airtight container in the refrigerator up to a week.