Preheat the oven to 325 degrees.
Make a graham cracker crust by mixing together the melted butter with the crumbs and 2 tablespoons sugar. Press into the bottom of a 9-inch springform pan.
Mix together the ricotta, the remaining ½ cup sugar, egg yolks, cream, espresso, coffee liqueur, lemon zest and salt until creamy. In a separate bowl, beat the egg whites until they form stiff peaks. Carefully fold the egg whites into the ricotta mixture. Stir in the chocolate chips and pour onto the prepared crumb crust.
Bake the cake for 1 hour and 15 minutes, or until just set in the middle. The cake will still seem wobbly but will be lightly browned and cracked around the edges. Remove the cake from the oven and cool on a rack for 30 minutes. Refrigerate the cheesecake for at least 6 hours or overnight before running a sharp knife along the edge of the cake and removing the pan. Cut into wedges to serve.