In a food processor, mix butter, milk, eggs, flour, cocoa powder, salt and sugar until smooth. Cover the batter with plastic wrap and refrigerate for an hour or up to 2 days.
To make the mousse filling, whisk whipping cream until firm peaks form. Fold in the melted chocolate and gently mix until incorporated. Chill in refrigerator.
To prepare crepes, heat a 6-inch skillet over medium-high heat with a light coating of oil. Scoop batter with a medium cookie scoop and pour into the skillet, swirling it until it evenly coats the bottom. Let it cook undisturbed until the bottom is golden and the top is set, about 30 seconds. Carefully flip to another side and cook for 15 to 20 seconds. Transfer the crepe to a baking sheet to cool. Continue with the remaining batter. It’s fine to stack the crepes to cool completely.
Using a 5-inch cookie cutter, cut out crepe rounds.
To assemble the cake, lay first crepe on the serving plate and spread with 2 tablespoons chocolate mousse. Repeat with all the crepes until you place the last crepe.
Chill the cake in fridge for 2 hours.
To prepare ganache, place the chocolate and heavy cream in a medium heatproof bowl over a pot of simmering water. Stir the mixture using a wooden spoon until melted and smooth. Let cool slightly.
Remove cake from the refrigerator. Transfer cake from cake pan to a serving plate.
Pour ganache over the top edges of the cake. Use an offset spatula to gently push the ganache over the sides of the cake to create the dripping effect.
Garnish with whipped cream and chocolate sprinkles.
Chill in the refrigerator. Let cake come to room temperature before serving.