Combine butter and sugar in a food processor. Pulse until no large pieces of butter remain. Add eggs, one at a time, and pulse until incorporated.
Add flour and cocoa. Process to combine. Drizzle in the cream and pulse until the dough comes together. Shape dough into a disc, wrap in plastic, and chill 2 hours.
After chilling, roll dough to about ¼" thickness and cut to fit tartlet pans (see note). After trimming the edges of the dough, dock the bottom and sides of each crust with a fork. Place tartlet pans on a baking sheet and refrigerate 30 minutes.
Preheat oven to 350F. Bake tart crusts until they appear dry and are slightly puffed, about 10 minutes. Allow crusts to cool completely.
For Caramel:
In a medium saucepan, combine sugar, water, and honey. Cook mixture over high heat, occasionally swirling the pan. Cook until mixture is dark amber in color, about 10 minutes.
Remove from the heat and (carefully!) add the cream (mixture will bubble!), whisking until smooth. Add butter, vanilla bean paste (see note), and salt. Stir to combine.
Pour about 1 teaspoon salted caramel into the bottom of each of the baked tartlet crusts.
For Hazelnuts:
Preheat oven to 350F (if not already preheated from baking the crusts). Place hazelnuts on a baking sheet. Toast hazelnuts for 10-12 minutes, or until slightly golden and fragrant.
Place the toasted hazelnuts in a kitchen towel and rub to remove skins. Allow hazelnuts to cool. Chop hazelnuts and divide them between the tartlets, sprinkling them on top of the caramel layer.
For Ganache:
Place chopped chocolate, and butter in a heatproof bowl. Set aside.
In a small saucepan, heat the milk to simmering. Pour over chocolate and butter, then whisk until smooth.
Spoon about a teaspoon of ganache into each tartlet on top of the caramel and hazelnuts. Use an offset spatula or gently tap each tartlet on your countertop to smooth the top of the ganache. Allow to cool and set before proceeding with the meringue.
For Meringue:
In a small heatproof bowl, combine egg whites, sugar, and salt. Place bowl over a pan of simmering water and warm until mixture is warm to the touch and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer. Using the whisk attachment, beat on medium-high speed until cool, voluminous, and shiny, about 10 minutes. Pipe meringue (or dollop with a spoon!) on top of each tartlet.
Using a kitchen torch (see note), toast the meringue. Enjoy!
Notes
Note on vanilla bean paste: I really enjoy the flavor of vanilla bean paste, and it's more economical (and lasts longer) than buying whole vanilla beans. I can never find vanilla bean paste at my local grocery store, so I buy it online [here]. If you prefer, you can substitute vanilla extract.Note on hazelnuts: I can never find them in my local grocery store. If you're in the same boat, you can find them online [here].Note on tartlet pans: I used teeny tiny tartlet pans (about 2.5" each), like the ones here). You could make one large (9" or 10") tart with this recipe, but baking the crust will take a few minutes longer.Note on egg whites: Egg whites need to be *completely* free of yolk! Make sure your bowl is squeaky clean, too. Even a drop of fat will keep your egg whites from whipping up properly. If you get yolk in your whites, dump it out, wash the bowl, and start over.Note on kitchen torch: I LOVE my kitchen torch, and find any excuse I can to use it. It's super handy, not to mention fun. If you're in the market for a kitchen torch, you can find one [here]. If you don't have one and prefer not to buy one, you can try toasting the meringue under your oven broiler. Keep a VERY close watch on it, as it's very easy to burn!