Melt butter in a saucepan and then add brown sugar and stir until smooth.
Bring to a slow bubbling boil for 1 minute. Remove from heat and allow to cool (but not long enough that the mixture gets hard..this step is very important).
Stir in eggs, flour, baking powder, salt and vanilla.
Mix well and spread into a well greased 8x10'' glass or ceramic in baking dish. Bake for 30 minutes.
While the bars bake, make the frosting.
For the frosting
Melt butter in a small saucepan over medium heat. Add brown sugar and stir until smooth.
Bring to a rolling boil and boil for 2 minutes.
Remove from heat and slowly stir in evaporated milk. Stir until well blended and then return to heat and bring to a boil once more.
Once boiling, remove from heat again and allow mixture to cool until you can touch the bottom of the saucepan with your hand.
Add powdered sugar--enough to reach a good frosting consistency. Spread frosting over warm bars and allow to cool before cutting the bars.
The frosting will set up as it cools.
To store, cover well with plastic wrap or place extra bars in an airtight container so they don't dry out.