Warm Fudge-Filled Cheesecake

1/2 cup butter or margarine, softened
1/3 cup granulated sugar
1 cup all-purpose flour
1 tablespoon vanilla extract, divided
2/3 cup chopped pistachios
32 ounces cream cheese (at room temperature)
1 1/2 cups granulated sugar
4 large eggs
1 (12 ounce) package semisweet chocolate chips
Sweetened whipped cream (optional)
Chocolate shavings (for garnish)

Beat butter at medium speed with an electric mixer until creamy; add the 1/3 cup sugar, beating well. Gradually add flour, beating at low speed until blended. Stir in 1 teaspoon vanilla extract and pistachios. Press onto bottom and 1 1/2 inches up sides of a 9-inch spring-form pan. Bake at 350 degrees F for 12 to 15 minutes or until golden. Cool on a wire rack.

Beat cream cheese at medium speed until light and fluffy; gradually add 1 1/2 cups sugar, beating well. Add eggs, one at a time, beating until yellow disappears. Stir in remaining 2 teaspoons vanilla extract. (Do not overmix.)

Pour half of batter into crust; sprinkle with chocolate morsels to within 3/4 inch of edge. Pour in remaining batter, starting at outer edge and working toward center. Place cheesecake on a baking sheet. Bake at 350 degrees F for 1 hour or until set. Cool on a wire rack 1 hour. Serve slightly warm with sweetened whipped cream.