Malted Milk and Dark Chocolate Tart

1 10-inch (25-cm) tart

• Heat the oven to 325°F (165°C) with a rack in the center. Spray a 10-inch (25-cm) springform cake pan with neutral nonstick cooking spray and set aside. To make the crust: In a mixing bowl, combine the flour, sugar, cocoa powder and salt. Mix to combine. In a separate container, mix the melted butter and discard sourdough starter. Add it to the dry ingredients, mixing with a spatula until all the dry ingredients are incorporated and the mixture is crumbly with bits ranging from pea- to marble-sized. Transfer the crumbly mixture to the prepared pan and gently press it into an even layer, covering the bottom of the pan but not going up the sides. The crust should be very lightly packed in; over-packing will make it hard to remove from the base when slicing. Refrigerate for 20 minutes.
• Once chilled, place the pan on a sheet pan and bake until set and crisp, 25 to 30 minutes. Cool at room temperature.
• To make the filling: Place the chocolate in a heatproof bowl and set it over a small, deep pot with about 2 inches (5 cm) of simmering water; make sure the bowl doesn’t touch the water. Stir occasionally until fully melted, about 5 minutes, remove from the heat, then add the cooled, melted butter. Stir to combine. Set aside to cool for 5 minutes.
• In a mixing bowl with hand beaters or using a stand mixer with the whisk attachment, beat together the granulated sugar, egg yolks and whole egg until light and fluffy, about 3 minutes. Drizzle in the melted chocolate-and-butter mixture and beat to incorporate. Add the sourdough starter, malt powder, vanilla and salt and mix until uniform. The batter will be loose and pourable, like a brownie batter. Pour the chocolate batter into the pan with the cooled crust, smoothing it out into an even layer with the spatula. Bake until set, 20 to 25 minutes. Remove the pan from the oven and allow it to cool to room temperature.
• To make the malted milk chocolate ganache: Set the milk chocolate in a deep, heatproof mixing bowl. In a small pot, combine the cream, malted milk powder, vanilla and a pinch of salt. Bring it to a boil, whisking to incorporate the malt. As soon as it starts to bubble up, remove it from the heat. Pour the cream mixture over the milk chocolate and let it sit for 5 minutes, then whisk vigorously until it comes together in a smooth, glossy ganache. Add the soft butter and whisk until fully incorporated. Cover the ganache and let it sit at room temperature to cool slightly and thicken up, about 30 minutes.
• When the tart is cooled to room temperature, stir the ganache and pour it over the top, giving the pan a gentle shimmy to help spread it out into an even layer. Transfer the tart to the fridge for at least 4 hours and up to overnight.
• When ready to serve, remove the tart from the fridge. Run a paring knife under very hot water. Quickly dry it off and run it around the edge of the tart to separate it from the sides of the pan. Release the ring of the springform pan and carefully remove it. Leave the tart on the base of the pan for serving and transfer it to a platter. Garnish with a light dusting of cocoa powder using a fine sieve and finish with flaky salt just before serving. Serve chilled or at room temperature.