Preheat your oven to 325°F (163°C). In a medium bowl, combine 1 1/2 cups cookie crumbs with 2 tablespoons of sugar and the melted butter. Stir until the crumbs are evenly coated. Press this mixture firmly and evenly into the bottom of your springform pan to form the crust. Bake for about 10 minutes. Remove from the oven and set aside to cool while you prepare the filling.
In a large mixing bowl, beat the cream cheese with 2/3 cup sugar until smooth and creamy using an electric mixer on medium speed. Add the mashed banana and sour cream, mixing well to combine. Then, add the flour and continue mixing until incorporated. Finally, add the eggs one at a time, mixing on low speed after each addition just until blended. Avoid overmixing to prevent cracks while baking.
Melt the unsweetened chocolate gently in the microwave or over a double boiler. Pour half of the cheesecake batter into the crust-lined pan. Drop spoonfuls of the melted chocolate over the batter, then pour the remaining batter on top. Use a butter knife or skewer to swirl the chocolate through the batter for a marbled effect.
Place the cheesecake in the oven and bake for about 50-60 minutes. The edges should be set, but the center will still jiggle slightly when you gently shake the pan. Turn off the oven and leave the cheesecake inside with the door slightly ajar for 1 hour. This gradual cooling helps prevent cracks.
Remove the cheesecake from the oven and cool to room temperature. Once cooled, refrigerate the cheesecake for at least 4 hours, preferably overnight. Before serving, top with freshly sliced strawberries, a generous dollop of whipped cream, and sprinkle with chopped walnuts if desired.