Make cake: Preheat oven to 350 degrees. In a food processor, blend almonds and cornstarch until finely ground, about 15 seconds. In a large bowl, using an electric mixer, beat egg yolks, sugar, lemon zest, vanilla, and salt until light and fluffy, 4 to 5 minutes. Gently fold in almond mixture. In another large bowl, whisk egg whites until stiff peaks form. Fold one-third the whites into almond mixture; fold almond mixture into remaining whites just until combined. Pour batter into a 9-inch springform pan. Bake until a toothpick inserted in center of cake comes out clean, 25 to 30 minutes. Let cool in pan.
Make pears and topping: In a large saucepan, bring 4 cups water, 3/4 cup sugar, cinnamon, and lemon zest to a boil. Reduce to a simmer, add pears, and cover pan with a piece of parchment to keep pears submerged. Cook until pears are tender when pierced with a paring knife, about 15 minutes. With a slotted spoon, transfer to a plate. Bring liquid to a boil and reduce until thick and syrupy, 15 to 20 minutes. Let cool to room temperature.
In a large bowl, whisk heavy cream until soft peaks form. Whisk in sour cream and 2 tablespoons sugar; continue to whisk until soft peaks return. Serve cake with pears, syrup, and whipped cream.