Preheat your oven to 350 degrees.
Using an electric mixer, cream together the cream cheese, sugar, eggs, & vanilla extract.
Lightly grease a muffin pan. Put an Oreo cookie in each one of the 12 muffin cups. Fill to the top with the cream cheese mixture.
Bake for about 20 minutes. Cool completely on a wire cooling rack.
If desired, melt the chocolate chips in a microwave safe bowl in 15 second intervals, stirring each time, until melted. Dip the bottoms of the mini cheesecakes in the melted chocolate, & cover the tops using a spoon. You can also just dip the bottoms, and drizzle the chocolate on top for a prettier presentation.
Set the chocolate covered cheesecakes on wax paper, or another non stick surface until set. Store in the refrigerator. Serve with a dollop of whipped cream on top if desired.