Combine the milk and espresso in a shallow bowl. Dip the biscuits briefly, just long enough to absorb flavor without losing structure. Arrange them snugly in a 14-by-9-inch/35-by-23-centimeter rectangular pan, forming two even layers.

Spread half of the cream evenly over the biscuits. Add another layer of dipped biscuits, then finish with the remaining cream, smoothing the surface. Refrigerate for at least 1 hour to set.

Place both chocolates in a heatproof bowl. Bring the cream to a full simmer, then pour it over the chocolate. Let stand for 1 minute, then stir gently until fully melted and glossy.

Pour the ganache over the chilled cake, tilting the pan to coat evenly. Return to the refrigerator and chill until fully set, at least 3 hours, before slicing and serving.