Grease a 10" tart pan with removable bottom (see note). Set aside.
In the bowl of a food processor, combine flour, cocoa, powdered sugar, and salt. Pulse briefly to combine.
Add diced butter and pulse until no large pieces remain. Beat egg yolk slightly and drizzle over the top. Process in long pulses until the dough comes together.
Turn the dough out onto a lightly floured surface and gently knead together. Roll the dough large enough to line your tart pan. Transfer to tart pan and trim edges. Freeze the crust for at least 30 minutes.
Preheat oven to 350° F. Grease some foil and line the frozen crust with it (you don't need pie weights because the crust is frozen). Place the tart on a baking sheet and bake 25 minutes. Remove the foil, then bake for another 7-9 minutes, or until the crust appears mostly dry. Remove to a wire rack to cool completely.
For Salted Caramel Sauce:
Pour sugar into a medium saucepan over medium-high heat. Whisk, and keep whisking, as sugar melts. It will clump and you'll think you're doing it all wrong. You're not. :)
Once sugar is melted, stop whisking and let sugar cook until it is deep amber in color. *Carefully* add butter (mixture will bubble!) and whisk to incorporate.
Remove mixture from the heat. Add cream (carefully!) and whisk until smooth. Add salt and stir to incorporate. Set aside to cool slightly. (Can be made in advance and stored in the refrigerator.)
Spread about a cup of the salted caramel sauce in the bottom of the cooled crust (see note). Set aside.
For Ganache:
Place chopped chocolate in a medium heatproof bowl. Set aside.
In a small saucepan, warm the coconut milk over medium heat until bubbling. Pour over chocolate and let sit two minutes.
Stir the ganache until smooth. Add vanilla and chocolate extracts (see note) and stir to combine.
Pour the ganache over the caramel layer. Chill at least two hours. Top with a drizzle of salted caramel sauce and a sprinkle of flaky salt, if desired.Enjoy!
Notes
Note on cocoa powder: I used dark (dutch process) cocoa powder for a darker color and slightly deeper chocolate flavor for the crust. If you only have regular unsweetened cocoa powder on-hand, that will do just fine.Note on tart pan: I used a 10" tart pan with removable bottom, like this one here.Note on salted caramel sauce: This recipe is a salted caramel *sauce*, so your tart will be a bit gooey and messy--which I don't mind. If you'd like it to hold it's shape a bit better, you can add less cream to the sauce. Of course you don't have to make your own and can substitute with store-bought caramel sauce if you'd like.Note on chocolate extract: I highly recommend this (sometimes overlooked) extract! It lends amazing depth of flavor to chocolate baked goods! If you can't find it in your local grocery store, you can find some here. Even better, make yourself some Homemade Chocolate Extract for next time!