Vigorously whisk the softened butter with the sugar.
Add the almonds, then fold in the eggs one at a time.
Mix well until the sugar melts.
Then, using a spatula, add the sifted flour – without overmixing – and 2cl of rum.
Gently butter, or better yet, line the cake pan with greased paper, pour in the mixture, and bake the cake at 170°C-180°C for 40 to 45 minutes.
Remove the cake from the mold, leaving the top part underneath, and as soon as it's out of the oven, drizzle with 2 cl of rum.
When the cake is cool, mix about 2 cl of rum with the icing sugar (to obtain a smooth consistency) and spread the glaze over the cake using a spatula.
Tip: Eat the Gâteau Nantais the next day; it'll be even better.