To prepare cupcakes: preheat oven to 350°F Line 9 muffin cups with paper liners. Beat butter and sugar in the bowl of an electric mixer until light and creamy. Add eggs and milk, then beat on medium-high speed for two minutes. Add flour, baking soda and salt; beat for an additional minute. Stir in toffee bits. Spoon into prepared tins and bake for 18 to 20 minutes or until a toothpick inserted in the center comes out clean. Remove from tins while still slightly warm.
Frosting: While cupcakes are baking, combine brown sugar, cream and butter in a heavy saucepan to prepare frosting. Bring to a boil over medium-high heat, stirring constantly. Continue boiling without stirring for one minute. Transfer to a mixing bowl and let cool for about 20 minutes. Beat in powdered sugar with an electric mixer until smooth; spread onto cooled cupcakes.