Preheat oven to 350°F.

Spray 2 13 ½ inch by 4 ½ inch false bottom tart pans with nonstick cooking spray.

Bring approximately 1 to 2 inches of water in a medium saucepan to a boil. Remove the pan from the heat. Place the oil, butter and 1 cup of the chocolate chips in a medium heatproof mixing bowl and set it over the saucepan of hot water to create a double boiler.

Allow the mixture to melt completely, stirring occasionally.

Remove the bowl from over the hot water.

Whisk the sugar into the melted chocolate mixture.

Add the eggs, one at a time, and whisk well after each addition.

In a separate medium bowl, whisk together the flour, salt and cocoa powder until well blended.

Using a rubber spatula, fold the flour mixture into the chocolate egg mixture in bowl.

Fold in the 2 cups mini semisweet chocolate chips and chopped walnuts. Divide evenly between the two greased tart pans.

Place filled tart pans on a large enough sheet pan so that they fit side-by-side lengthwise.

Bake tarts for 28-30 minutes or until dull and crackly looking.

Cool completely. Chill in the refrigerator until firm.

Remove the tarts from the pans and if desired, drizzle with the melted chocolate. Allow chocolate to harden before slicing into pieces.