Sift or whisk flour, baking powder, salt, cocoa powder, and espresso powder together in a mixing bowl and set aside.
In a stand mixer with the whisk attachment add the eggs and sugar and beat on medium-high until the mixture is pale is color and has at least tripled in volume. Then beat in the vanilla.
Using a rubber/silicone spatula gently fold in the flour mixture. Once incorporated gently fold in the melted butter. Lightly press cling film onto the top of the batter to prevent a skin from forming and refrigerate for at least 3 hours and up to a day.
Preheat the oven to 400°F or gas mark 6.
Lightly spray your madeleine pan with baking spray or brush with a small amount of melted butter. Add a heaping tablespoon into each well. Bake for 11-13 minutes or until the bump springs back when touched gently. Remove from the oven and tap on the counter to loosen the madeleines. Transfer them to a cooling rack to finish cooling. It is important to remove the madeleines quickly as they might adhere to the pan if left to cool too long.
Adding the chocolate coating
Melt the chocolate and transfer into a narrow glass, this makes it easier for dipping.
Once the madeleines have cooled to room temperature dip whichever side you prefer to coat in chocolate.