The optional cornstarch in the filling exists solely to help stop the spread of the caramel filling after you cut it. If you don’t mind that feature of the classic recipe, just omit it. With an unsweetened cocoa crust and barely sweet caramel, I prefer a sweeter chocolate, preferably around 60 percent cocoa solids, but use any bittersweet chocolate you want, going up to even 75 or 80 percent if you can handle the intensity.
Make the crust: In a medium bowl, whisk together the flour, cocoa powder and salt. In a small saucepan or skillet, melt the butter over medium heat, then stir in 2 tablespoons water and return the mixture to a simmer. Immediately pour the hot butter and water over the dry ingredients and stir with a spoon until it forms a clumpy dough. Clean off the spoon with your hand and knead the dough in the bowl just until it’s smooth and shiny — don’t worry, it will be warm but not too hot to handle.
Immediately transfer the dough ball to a 9-inch round springform pan or tart pan (to use a cake pan, see note below). Press the dough into the bottom and 1/2 inch up the sides of the pan, smoothing it out with your fingers and creating an even wall around the perimeter. Place the pan in the freezer and chill until the dough is firm, at least 30 minutes.
Heat the oven to 400 degrees. Transfer the pan directly from the freezer to the oven and bake until the crust is set, about 20 minutes. You won’t be able to tell it’s done by color, so look for the crust to have slightly pulled away from the side of the pan and to look matte. Transfer the pan to a wire rack and let the crust cool while you make the caramel.
Make the caramel filling: If using the cornstarch, place it in a liquid measuring cup, pour in a splash of cream and stir until smooth. Pour in the remaining cream until it reaches the 1/2-cup mark. In a small bowl, stir together the vanilla and salt.
Pour 1/4 cup water in a small, heavy-bottomed saucepan, then pour in the sugar and corn syrup. Place the pan over medium heat and stir to dissolve the sugar, about 3 minutes. Stop stirring, increase the heat to medium-high and continue cooking the syrup, swirling the pan occasionally, until it turns the color of golden honey, 5 ½ to 6 ½ minutes.
Immediately remove the pan from the heat and use a long-handled rubber spatula or wooden spoon to stir in the butter until melted and smooth (be careful, it will bubble and sputter). Stir the cream mixture once more, then pour it into the pan. Let the bubbling subside, then pour in the vanilla and salt and stir everything together until smooth. Pour the caramel into the tart crust (it doesn’t matter if it’s not cooled completely) and tap the pan to ensure it evenly settles. Place the pan in the refrigerator and let the caramel filling chill until firm, at least 2 hours.
Make the ganache: Snap the chocolate bar into small 1/2-inch pieces (or roughly chop with a knife) and place in a small, tall bowl. Pour the cream and corn syrup into a small saucepan and bring to a bare simmer over medium heat. (Alternatively, heat the cream and corn syrup in a heat-proof glass measuring cup in the microwave until it starts to bubble.) Pour the hot cream over the chocolate and tap the side of the bowl to settle the chocolate underneath the surface of the cream. Let the mixture stand, untouched, for 1 minute.
Using a small rubber spatula or spoon, slowly but continuously stir the cream in the center only until it emulsifies into a shiny dark chocolate ganache, then move your spatula to stir and incorporate all the cream, stirring until there are no more pieces of solid chocolate still visible. Pour the ganache over the chilled caramel filling and spread it to the edge of the crust with the spatula.
While the ganache is still wet, sprinkle it liberally with flaky sea salt, making sure not to neglect the edges. Let the tart stand at room temperature for at least 2 hours, or in the fridge for 1 hour, to allow the ganache to set before serving.
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