For the crust:
5 ounces amaretti cookies, processed in a food processor to uniformly fine crumbs (about 1 ¼ cups)
4 tablespoons melted butter, plus 1 tablespoon melted butter for greasing pan
For the ricotta filling:
2 pounds ricotta cheese, drained overnight (see note below)
4 large eggs, separated
¾ cup sugar
1 tablespoon unbleached all-purpose flour
Grated lemon zest from 1 lemon
1/8 teaspoon kosher salt
2 teaspoons vanilla extract
Orange marmalade for serving, if desired
Before making the cheesecake, drain the ricotta cheese overnight or for at least 8 hours. Line a fine mesh sieve with two layers of paper towels, place the cheese in the sieve that has been set over a bowl. Place in the refrigerator while the cheese drains.
For the crust:
Adjust oven rack to lower-middle position and heat oven to 325 degrees. In a small bowl, combine cookie crumbs, 4 tablespoons melted butter and toss with a fork until evenly moistened. Brush bottom and sides of a 9-inch springform pan with most of remaining 1 tablespoon of melted butter, reserving small amount of excess butter for brushing sides of pan after crust cools. Press the crust firmly and evenly into the springform pan. Bake for 13 minutes, until crust is fragrant and golden brown. Cool on a wire rack until room temperature, brush sides with remaining butter.
For the filling:
While crust cools, places drained ricotta in a food processor and process until very smooth, about 1 minute. Add yolks, sugar, flour, zest, vanilla and salt and process until blended, about 1 more minute. Scrap mixture into large bowl.
In the bowl of a stand mixer, beat egg whites at high speed until they hold stiff peaks. Fold whites into ricotta mixture until fully incorporated and pour mixture evenly into cooled crust.
Bake cheesecake until top is lightly browned and instant-read thermometer inserted into center registers about 150 degrees, about 1 ¼ hours. (Perimeter of cake should be firm, but center will jiggle slightly. It will solidify further as cake cools.) Transfer pan to a wire rack and cool for 5 minutes. Run paring knife between cake and side of springform pan. Cool until barely warm, 2 ½ - 3 hours. Wrap pan tightly in plastic wrap and refrigerate until cheesecake is cold and set, at least 5 hours or up to 2 days.
To unmold cheesecake, remove sides of pan. Let cheesecake stand at room temperature for about 30 minutes, then cut into wedges and serve.