Prepare crust: Heat oven to 350°. Combine 1.25 cup crushed vanilla wafers, 1/3 cup powdered sugar, and 1/3 cup Hershey's cocoa. Stir in 0.25 cup melted butter. Press mixture onto bottom and 0.5 inch up side of 9-inch springform pan. Bake 8 minutes. Cool completely. After baking the crust, heat oven to 450°.

Prepare batter base: Beat cream cheese, 0.75 cup sugar, sour cream, and 2 tsp vanilla in a large bowl on medium speed until smooth. Gradually add flour, beating until just blended. Add eggs, one at a time, beating well after each addition.

Prepare chocolate batter: Combine cocoa and remaining 0.25 cup sugar in a medium bowl. Add oil, remaining 0.5 tsp vanilla extract, and 1.5 cup of batter base. Stir well. Spoon plain and chocolate batters alternately on prepared crust, ending with spoonfuls of chocolate on top. Gently swirl with knife for marbled effect.

Bake: Bake 10 minutes. Without opening oven door, reduce oven temperature to 250° & bake for another 30 minutes. Then, turn off the oven & keep cheesecake in the oven for an additional 30 minutes.

Cool: Remove from oven & immediately loosen cheesecake from side of pan with a knife. Cool to room temperature, then refrigerate overnight. Remove side of pan & cover cheesecake.