FOR ICE CREAM: Place ice cream in medium bowl and soften slightly at room temperature.
Stir in chopped toffee bars and over bowl with plastic wrap and freeze. (Can be prepared 3 days ahead)
FOR CARAMEL SAUCE: Bring whipping cream to simmer in heavy small saucepan and set aside.
Combine sugar and 3 tablespoons water in heavy medium saucepan.
Stir over medlium heat until sugar dissolves and increase heat to high and boil without stirring until caramel is deep amber color.
Brushing down sides of pan with wet pastry brush if sugar crystals form and occasionally swirling pan, about 8 minutes.
Remove from heat and slowly add warm whipping cream (mixture will bubble vigorously).
Return sauce to low heat and stir until smooth.
Remove from heat and mix in butter. (Can be prepared 3 days ahead. Cover and refrigerate.)
FOR BROWNIES: Preheat oven to 350 degrees.
Butter 9" square baking pan with 2" high sides.
Combine butter and both chocolates in top of double boiler set over simmering water; stir until melted.
Remove from over water and cool mixture slightly.
Using electric mixer, beat eggs, sugar and vanilla in large bowl until slightly thickened, about 1 minute.
Beat in chocolate mixture.
Add flour, baking powder and salt and mix until combined.
Stir in all but 2 tablespoons chopped toffee bars and transfer batter to prepared pan.
Sprinkle with remaining 2 tablespoons chopped candy.
For fudgy consistency, bake until tester inserted into center comes out with moist batter still attached, about 27 minutes; for cakelike texture bake until tester inserted into center comes out with a few moist crumbs attached, about 35 minutes.
Cool. (Can be prepared 1 day ahead. Cover and store at room temperature.)
Rewarm sauce over low heat and cut brownie into 9 pieces.
Spoon warm caramel sauce onto 8 plates.
Place 1 brownie in center of each (reserve extra brownie for another use) and surround each with 3 small scoops of ice cream.