- Preheat the oven to 325°F (160°C). Grease the bottom and sides of a 9-inch (23 cm) springform pan and wrap the outside of the pan in foil.
- For the crust, stir the chocolate cookie crumbs and melted butter together until evenly blended. Press into the bottom of the prepared pan. Bake for 10 minutes to set it (there will be no visible change once baked), then cool while preparing the filling.
- For the cheesecake, melt the chocolate by placing it in a metal bowl set over a pot filled with 1 inch (2.5 cm) of barely simmering water, stirring gently with a spatula until smooth. Set aside while preparing the cheesecake batter.
- Using electric beaters or a stand mixer fitted with the paddle attachment, beat the cream cheese at medium-high speed until smooth, scraping down the bowl once or twice. Add half of the granulated sugar, then beat at medium-high and scrape down the bowl again. Add the remaining granulated sugar and the brown sugar and beat again, once again scraping down the bowl. Beat in the cocoa powder and salt at low speed, then beat in the sour cream and vanilla, again scraping the bowl to ensure the batter is smooth (it will be quite fluid now).
- Add the eggs and egg yolks one at a time, while mixing at medium-low speed, until fully combined. Pour in the melted chocolate (still warm is OK) and beat at medium-low until smooth.
- Place the springform pan in a large roasting pan and pour the batter into the pan. Fill the roasting pan with boiling water so that the water comes to at least halfway up the sides of the pan (this is easiest done at the oven). Bake the cheesecake for about 65 minutes until it loses its surface shine and jiggles in the centre.
- Transfer the pan from the water bath to a cooling rack. After letting the cake cool for 10 minutes, run a palette knife around the inside edge of the springform pan (this helps prevent cracks from developing while the cake cools). Let the cheesecake cool completely (cooling can take up to 2 hours) before chilling, uncovered, overnight.
- Release the sides of the springform pan and transfer the chilled cheesecake to a platter or cake stand.
- For the ganache, heat the cream and butter in a saucepan over medium-high heat until it just begins to bubble. Pour this mixture over the chopped chocolate, let it sit for a minute and then gently stir with a spatula until the chocolate has melted and the ganache is smooth.
- Pour the ganache over the centre of the cheesecake. Using an offset spatula, quickly but gently spread the ganache to the edges of the cake, coaxing it just over the top edge to drip down the sides a little (the chilled cheesecake will set the ganache relatively quickly). Chill the cheesecake, uncovered, until ready to serve. Once cut, cover the cut surface directly with parchment or plastic wrap and it will keep in the fridge for up to 4 days.
From Baking Day with Anna Olson, Appetite by Random House, 2020.