Preheat oven to 350F/180C. Wrap the bottom and sides of a 9-inch springform pan with one large piece of foil, then repeat with another piece. This will ensure that water from the water bath doesn’t seep into the cake.
In a medium bowl mix crumbs and melted butter until evenly combined and moistened. Press mixture into pan. You can use the bottom of a glass to press it into an even layer. Bake for 8-10 minutes, until golden brown. Allow to cool completely on a wire rack before adding the filling. Meanwhile, prepare the filling.
Using an electric mixer fitted with the paddle attachment, beat cream cheese on medium-low speed until smooth and lump-free, 1-2 minutes. Add ricotta and sugar and beat until combined and cheese is smooth. Beat in eggs, one at a time, until fully combined. Scrape down the sides and bottom of the bowl as necessary. Add cornstarch, vanilla extract, and zest, and beat just until blended and smooth. Pour batter over cooled crust and spread evenly.
Place pan inside a large roasting pan, then pour enough boiling water into the roasting pan to come halfway up the sides of the cake pan. Place carefully in the oven and bake for 60-70 minutes or until sides of the cheesecake are set but the center is still slightly wobbly.
Transfer pan to a wire rack and allow to cool completely. Cover cheesecake with plastic wrap and refrigerate for 8 hours to set. The cake will taste best a day after it’s made.
Cake can be kept in the fridge, well covered, for up to 5 days.