Grease a 23cm loose-bottomed tart tin with butter and dust with flour.
Sift flour and icing sugar into a bowl, then rub in chilled butter to reach a breadcrumb consistency.
Create a well, add eggs, and mix by hand into a sticky dough.
Knead dough until smooth, flatten to 1cm, cover, and chill for at least 30 minutes.
Boil prunes with 150ml Armagnac, 100g caster sugar, and 100ml water, then soak for 10 minutes until plump.
Roll pastry to a 3mm thickness on a floured surface.
Line the tart tin with pastry and press into the edges carefully.
Purée half the prunes with 3 tablespoons of soaking liquid and spread over the pastry base.
Preheat oven to 190°C.
Cream butter and remaining caster sugar, beat in eggs one by one, then fold in flour, almonds, and remaining Armagnac.
Fill the pastry case, decorate with remaining prunes and whole almonds, reserving any leftover liquid.
Bake for 25-35 minutes until golden and puffed.
Boil the reserved prune liquid for 5-10 minutes until it becomes sticky syrup.
Trim pastry edges and brush the tart top with the syrup.
Whisk double cream to soft peaks and fold in a splash of Armagnac.
Dust with icing sugar and serve warm with the cream.