Place butter and sugar in the bowl of a mixer and mix on medium for 3 minutes. Scrape down the sides and add the egg yolk and beat well.
In another bowl whisk together the flour and cocoa powder. Add to the butter mixture all at once. Then mix in at lowest speed until it just comes together (it will have small and medium clumps). Any more and it will become sticky and difficult to work with.
Scrape dough into mini tart pans. Press a thin layer into the bottom and sides of the tarts. Chill for an hour in the fridge.
Preheat the oven to 350F. Bake tart shell for 20 minutes, watching carefully the last 5 minutes to make sure it doesn't burn. Cool on wire rack.
Caramel: In a saucepan, whisk together sugar, corn syrup, and water and bring to a boil. Cook over medium-high heat until a medium-dark amber color, swirling occasionally. Be very careful - caramel can go from "not quite dark enough" to "burnt" in a matter of seconds - don't leave the pan unattended!
Take off heat and add the cream, butter, and sour cream. Stir together until smooth.
Sprinkle toasted pecans into cooled tart shells, pour caramel on top until shells are about 3/4 full, then sprinkle kosher salt on top of caramel. Let cool.
Ganache: Bring cream to a boil in a 1-qt. saucepan over medium heat. Put chocolate into a medium bowl and pour in hot cream; let sit for 1 minute, then stir slowly with a rubber spatula until smooth. Pour ganache evenly over tarts, garnish with more pecans if desired, and refrigerate until set, 4 to 5 hours. Serve chilled.