Combine the melted butter, brown sugar and vanilla.
Mix in the eggs.
In a separate bowl mix flour, salt, and baking powder.
Add the flour mixture to butter mixture and stir until combined.
Stir in walnuts
and white chocolate.
Spoon mixture into a greased 13 x 9 inch pan
or a well-buttered 12 inch skillet.
Bake at 350 degrees for 20-25 minutes. Slightly underbaked is really delicious!
If baking in a skillet it may need 5 or so extra minutes.
Let cool and cut into bars.