Preheat oven to 350 degrees. Spray a 9-inch tart pan with non-stick spray. Combine flour, cocoa, pecans, and sugars in the bowl of a food processor. Pulse until evenly blended and pecan pieces are no larger than the size of small peas. Add the melted butter and pulse until mixture is moistened and sticks together when pressed between your fingers. Press crust into the bottom and up the sides of the prepared tart pan. Bake 10 minutes. Let cool completely on a rack.
For the chocolate mousse filling:
In a heatproof bowl over a saucepan of barely simmering water, combine the chocolate, butter, bourbon, and water. Allow the butter and chocolate to melt, then whisk mixture until smooth and glossy. Remove from heat and add the egg yolks, one at a time, whisking after each addition to combine. Add vanilla. Continue stirring until mixture is smooth and glossy again. Set aside to cool, stirring frequently with a spatula for another 5-10 minutes until just warm.
Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment or in a mixing bowl with a hand mixer, whip the cream to medium-firm peaks. Set aside. In another clean mixing bowl, combine room temperature egg whites and salt. Using a clean whisk attachment or beaters, beat egg whites on medium-high speed until soft peaks form. It takes a bit longer to whip pasteurized eggs whites, and adding a bit of cream of tartar or lemon juice will help. Slowly add sugar and beat until glossy, firm peaks form.
Spoon the whipped cream onto the cooled chocolate mixture and fold in gently with a rubber spatula, stopping when large streaks of chocolate are still visible. Add the egg whites and continue folding until just combined. Pour mousse mixture into cooled pecan crust and cover with plastic wrap. Chill in refrigerator for at least 4 hours or overnight before drizzling with caramel and serving.
For the caramel sauce:
In a small non-stick saucepan, bring the sugar, water and corn syrup to a boil over high heat. Cook until the sugar is dissolved, washing down the side of the pan with a wet pastry brush to prevent sticking and burning. Continue cooking, without stirring, until an amber caramel forms, about 6 minutes. Remove from the heat and carefully stir in the cream (the mixture may violently bubble once cream is added, that's okay). Let cool for 1 minute, then stir in the bourbon. Place back on medium-high heat and bring the mixture to a boil and cook, stirring, for 1 minute. Remove from heat and let the caramel sauce cool slightly to room temperature. Drizzle over chocolate mousse tart and serve. Caramel can be made up to 4 days ahead of time and stored in an airtight container in the fridge.
The assembled mousse tart can be stored in the fridge for up to 3 days.
Notes
To toast the pecans for the crust, spread them evenly on a baking sheet and bake in a 350 degree oven for 5 minutes, or until fragrant. Be careful not to burn!*Any alcohol in this recipe is optional and simply adds more flavor. Add as little or as much as you prefer. You could substitute another liqueur such as a raspberry, orange, or coffee liqueur in the mousse or even the caramel.**Be sure the egg whites are at room temperature before whipping; this allows them to better increase in volume.