Combine chocolate pieces, cream cheese, and evaporated milk in a saucepan.

Stir over low heat until smooth.

Stir in chopped walnuts.

Preheat oven to 375F (190C).

In a large bowl, cream sugar and margarine until light and fluffy.

Blend in eggs and vanilla extract.

Add flour, baking powder, and salt; mix well.

Press half of the flour mixture onto the bottom of a 12 x 9 x 2-inch baking pan to form the crust.

Spread chocolate mixture evenly over the crust.

Sprinkle with the remaining crust mixture.

Press down gently.

Bake for 30 minutes, or until the crust is lightly browned.

Cool completely before cutting into bars.

To freeze, prepare as directed and wrap securely before freezing.

When ready to serve, thaw unwrapped at room temperature for approximately 2 hours.