(from the fantastic "Crumb" by Ruby Tandoh)
150g graham crackers
2 tablespoons superfine sugar
Juice of half a lemon
1 teaspoon cornstarch (optional)
2 tablespoons cornstarch
1/2 cup and two tablespoons heavy cream
125g superfine sugar
2 large eggs and one large egg yolk
zest of one lemon
1.5 teaspoons vanilla extract
Preheat the oven to 170°C. Line a 20cm springform pan with parchment paper.
To make the crust, crush the graham crackers with a rolling pin or in a food processor until sandy. Melt the butter over low heat, then mix it with the graham cracker crumbs. The crumbs need to be moist enough that they'll hold together in clumps if squeezed, so add a bit more butter at this point if necessary.
Spread the mixture over the bottom of the pan and press down firmly all over. Bake for 10 minutes, then remove and let cool slightly. Decrease the oven temperature to 120°C.
Then, make the blackberry swirl. In a small pan, heat the blackberries, sugar and lemon juice over low heat, stirring occasionally until the blackberries soften. Crush with the back of a fork until no whole berries remain, then set aside to cool. If the blackberries release a lot of juice, add a teaspoon of cornstarch while the pan is still on the heat an stir for another minute or two, until thickened.
To make the filling, beat the ricotta and cornstarch together in a large bowl until creamy. Then, stir in the cream. When combined, add sugar, eggs and yolk (one at a time), lemon zest and vanilla extract.
Pour the filling onto the baked crust. Dollop spoonfulls of the blackberries on top and gently swirl through the filling using a spoon. Bake for 1 3/4 to 2 hours (with my oven, it took one and a half hour) until the filling has no more than a slight wobble in the center. The cooked cheesecake should be quite firm all over except for the blackberry swirls, which will now be sunken, jammy and a deep purple color.
Let the cheesecake cool completely in the pan beroe unmolding. Then, for a firmer texture, chill in the fridge before serving.