Heat the oven to 180C/160C fan. Use the pastry to line a 23cm loose-bottomed fluted tart tin, press it into the edges, trim the overhang and chill the case in the fridge for 15 minutes.
Beat the softened butter and caster sugar together until pale and fluffy, then beat in the ground almonds, followed by the eggs, almond extract and salt. Finally fold in the flour. The mixture will look like a thick, slightly grainy cake batter.
Spread the jam over the base of the chilled pastry case in a thin, even layer. Sharp or seedless jam works best here because sweetness is coming from the filling too.
Spoon the almond mixture over the jam and spread it gently to the edges without disturbing the jam layer beneath. Scatter the flaked almonds on top if using.
Bake for 35-40 minutes until the filling is risen, deep golden brown and firm to a gentle press in the centre with the faintest wobble left.
Cool in the tin for at least 20 minutes before removing and slicing, the frangipane needs to set or the slices will smear. Excellent warm, arguably better cold, and a small slice goes a long way.