Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Line a baking dish of approximately 21x32 cm/ 9x13 inch with baking paper.

Separate the eggs. Set the yolks aside for the buttercream.

Beat the egg whites with a pinch of salt until stiff. Slowly add the sugar and continue beating until the egg whites are stiff and glossy.

Fold in the poppy seeds.

Mix together the flour and the baking powder in a small bowl. Fold them into the egg white-poppy mixture.

Bake for about 30-35 or until a toothpick inserted in the middle comes out clean. Leave to cool completely in the tin. In the meantime make the vanilla buttercream.

Place the egg yolks and the sugar in a saucepan and mix well. Add the milk and the vanilla pudding powder (or corn starch and vanilla extract) and whisk very well to prevent getting clumps.

Heat the mixture gently while whisking all the time until the mixture begins to thicken. It will take about 10 minutes, but please whisk continuously or it will clump. Cover the pudding with plastic foil directly over the surface, this way it will not form a “skin”.

Let the vanilla pudding reach room temperature. Take the butter out of the fridge and let it reach room temperature as well.

Cut the soft butter into small cubes. Gradually add the butter cubes to the vanilla pudding and mix well to incorporate.

Spread the buttercream evenly over the cooled base in the tin.

Brew a cup of strong coffee. Dip the biscuits one by one into the coffee, but don't let them get soaked, they should be just barely wet.

Arrange the biscuits on top of the vanilla buttercream.

Chop the chocolate very finely and place it into a small heatproof bowl. Add the chopped butter and the heavy cream. Melt the chocolate on bain-marie, stirring often until everything is melted and mixed.

Let the chocolate glaze cool for a few minutes and pour it over the biscuits while still warm. Spread it evenly on the cake and place the cake in the fridge for at least half an hour until the glaze sets.

Cut into small squares while still in the tin. Carefully lift the squares out of the tin and serve.