Step 1 For the crust: Adjust an oven rack to the middle position. Preheat the oven to 325°F.
Step 2 Put the graham cracker crumbs and sugar in a medium bowl and whisk. Pour the melted butter over the top and stir with a spatula until combined. Press the mixture into an 8 or 9 in [20 or 22 cm] springform pan and bake 10 to 12 minutes, or until the crust is golden. Remove from the oven and set aside to cool.
Step 3 For the cheesecake: In a stand mixer fitted with a paddle, beat the cream cheese on high until smooth and creamy, about 3 minutes. Add the sugar, vanilla, salt, and lemon juice and beat on medium until light and smooth, about 3 minutes. Transfer the cream cheese mixture to a large bowl and set aside. Clean the mixing bowl if you have only one.
Step 4 In a stand mixer fitted with a whisk, beat the cream (or the heavy cream and the Skyr) on low for 30 to 45 seconds. Increase the speed to medium and continue beating until stiff peaks form, 1 to 2 minutes.
Step 5 With a rubber spatula, stir about one-third of the whipped cream into the cream cheese mixture and fold until completely combined. Add the remaining whipped cream and fold until combined and no streaks remain.
Step 6 Scrape the filling over the cooled crust and smooth the top with an offset spatula. Using a wooden skewer or knife, decorate the top of the cheesecake with jam swirls. Refrigerate the cheesecake until firm, 8 hours or overnight.
Step 7 For the jam swirls: Combine ¼ cup raspberry jam and 1 teaspoon water in a small saucepan. Warm over low heat just until the jam is loose. Let the mixture cool to room temperature. Drop the jam in circles over the top of the cheesecake and use a skewer or knife to drag it through the top of the cheesecake, making swirls. Refrigerate as directed.
Step 8 For Chocolate Ganache topping: Place 6 ounces [170 g] semisweet chocolate, chopped fine, in a small bowl. Heat ½ cup [120 g] heavy cream in a small saucepan until it is simmering and just about to boil. Pour the cream over the chocolate, cover the bowl with plastic wrap, and let sit for 5 minutes. Remove the plastic and whisk until completely smooth. When the cheesecake is firm, top with the cooled ganache, and let set. Slice and serve.
