MIX brown sugar, cornstarch and salt in a large microwave-safe bowl. Whisk in milk and egg yolks until well blended.
MICROWAVE on HIGH (100%) power for about 5 to 7 minutes, stirring every 1 to 2 minutes. Cook until pudding is thick and has boiled at least 1 minute. Remove from oven. Stir in butter and vanilla. Cover surface with plastic wrap.
CHILL 30 minutes.
STIR in toffee chips. Pour into a bowl or individual serving dishes. Garnish with pecans and whipped topping, if desired.