Preheat the oven to 350 degrees F. Line a 13- by 9-inch baking pan with foil. Spray with nonstick cooking spray.
In a small saucepan, combine the unsweetened chocolate and butter. Cook over low heat, stirring frequently, until the chocolate melts and the mixture is smooth. Let it cool for 15 minutes.
In a large bowl, beat the sugars, vanilla and eggs at medium speed with a mixer until fluffy. Gradually beat in the flour. Add the chocolate mixture to the flour mixture, beating until combined. Spoon the batter into the prepared pan.
Bake until the center is set, 25 to 30 minutes. Let cool in pan for 1 hour.
In a small saucepan, combine the semisweet chocolate and cream. Cook over low heat, stirring frequently, until the chocolate melts and the mixture is smooth. Spread over the cooled brownies, cover and chill for at least 4 hours or up to 3 days.
Spread Praline Frosting over the chocolate mixture. Let stand until set. Cut into squares to serve.
In a medium saucepan, melt the butter, and then add the brown sugar and mix together. Slowly add in the cream. Bring to a boil over medium heat and boil for 1 minute. Remove from the heat and stir in the confectioners' sugar, pecans and vanilla extract. Continue stirring until the mixture thickens, 3 to 4 minutes. Spread immediately over chilled brownies.