Cream the butter until light and fluffy. Gradually incorporate the sugar, beating for several minutes until the mixture becomes frothy and pale. Fold in the finely ground walnuts.
Prepare the custard using the milk, egg yolks, and caster sugar. Allow it to cool completely, then gently fold it into the butter-walnut mixture.
Line the bottom and sides of a charlotte mold with spoon biscuits, positioning the rounded sides against the mold.
Spoon the nut mixture into the center, filling evenly.
Top with another layer of biscuits, flat side facing outward. Refrigerate overnight for the best texture.
To serve, unmold the charlotte and prepare the glaze: Melt the chocolate with the water and remaining butter in a bain-marie. Stir until smooth and glossy.
Pour the glaze into the center and tilt the mold gently to coat the top, allowing it to drape over the edges.
Source: Les Bons Desserts
