Preheat. Set the oven to 350°F (177°C).
Make the crust. In a large bowl, combine the crushed chocolate wafer cookies, melted butter, sugar, and salt. Press into the bottom and about 1 inch (2.5 cm) up the sides of a 9-inch springform pan. Bake for 10 minutes, then reduce the oven temperature to 325°F (163°C).
Make the filling. While the crust bakes, beat the cream cheese, sugar, lemon juice, and vanilla together until smooth. Add the flour, then add the eggs one at a time, mixing just until combined.
Colour a portion of the batter. Transfer 2 cups of the batter to a medium bowl and stir in the cocoa powder and red food colouring until evenly coloured.
Layer and swirl. Pour the red batter into the crust, followed by the plain batter on top. Using a knife, gently swirl the two together — just a few strokes, so the colours stay marbled rather than blending into one.
Bake. Bake for 80 minutes, until the centre is almost set. Turn off the oven and let the cheesecake cool inside, undisturbed, for 20 minutes.
Chill. Cover and refrigerate overnight.
Garnish and serve. Top with cream cheese frosting just before serving.