Preheat the oven to 350 degrees. Line a standard muffin pan with paper liners.

In a medium bowl, sift together the cocoa, flour, baking powder, espresso powder, and salt. Set aside.

Place the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Cream together on medium high until light and fluffy, about 3 minutes. Add the eggs on low speed one at a time, mixing until well incorporated after each addition. Mix in the vanilla. Add the flour mixture in 2 batches, alternating with the Greek yogurt, ending with the flour mixture. Mix just until combined, scraping down the sides of the bowl with a rubber scraper as needed.

Fill the cupcake liners 3/4 full, and bake on the middle rack of the oven for 20-25 minutes, or until the tops spring back when touched lightly or a toothpick inserted into the center comes out clean. Cook in the pan on a wire rack for 5 minutes, then remove from the pan and cool completely.

Place the sugar and water in medium saucepan over medium low heat. Cook, stirring, until the sugar has dissolved. Increase the heat to high, and boil the sugar without stirring. If the sugar begins to crystallize on the sides of the pan, carefully use a wet pastry brush to brush it off. Boil until the sugar turns a deep amber color, being careful not to burn, about 5 minutes. Remove from heat and slowly whisk in the cream a bit at a time, as the caramel will bubble over if too much is added at once. Whisk in the butter and fleur de sel. Transfer to a dish to cool.

Place the butter and powdered sugar in the mixing bowl of a stand mixer fitted with the paddle attachment. Beat on medium until the sugar has been incorporated. Add the milk, caramel sauce and fleur de sel, and beat until combined. Add the melted chocolate, and beat until incorporated. Once incorporated, beat the buttercream at medium high speed for 3 minutes until creamy.

Pipe the buttercream onto the cupcakes, and garnish with more caramel sauce and sprinkle with fleur de sel. Enjoy!