If you are making your own pastry dough, aerate the flour by sifting it together with the salt and sugar into a large bowl. Add butter in pieces and rub it in with your fingertips until you reach a texture similar to fine bread crumbs. (Try to keep your hands cool). Add cold water gradually, mixing only enough water as you need to make the dough form. Form the dough into a disk (for ease of handling), and place it in a bowl covered with plastic or a clean damp towel, and allow to rest in the refrigerator for 30 minutes. When rested, roll the pastry out onto a lightly floured surface with a rolling pin, to a thickness of about 1/8-inch and line a well buttered 9 1/2-inch flan pan with a removable bottom. Trim away any excess pastry hanging over the edge of the pan. Place pastry shell into the refrigerator for about 30 minutes.
Preheat the oven to 375 degrees F.
Line the base of the chilled pastry shell with parchment paper. Then fill with ceramic pastry weights (or dried beans) and place on a baking tray in the oven and partially bake ("blind bake") for 25 minutes. Remove the weights/beans, prick the base of pastry to prevent bubbling-up of base, and return to the oven for a further 5 minutes to dry out the base. Remove pastry shell from oven and allow to cool. Also allow oven to cool to 350 degrees F.
While the pastry shell is cooling, prepare the sponge cake batter:
Heat the milk over low heat. Add the butter and let it melt into the milk. Set the pot aside temporarily. Beat the sugar and eggs in a large bowl with an electric beater until it is about triple in volume and is the consistency of soft whipped cream. Return the pot with the milk to the stovetop and heat until hot. Sift cake flour and baking powder together several times to aerate. Sift the 1/3 of the flour and baking powder into the bowl of beaten eggs and sugar, and fold together. Repeat for each remaining third of the flour mixture, folding together after the addition of each third. Then, add the hot milk and butter all at once and fold in.
For the filling and topping: Preheat the oven to 350 degrees F.
Spread the jam over the base of the cooled pastry shell and pour the cake batter over the top of the layer of jam. Then sprinkle the surface of the cake batter with coconut. Bake in oven for 20 to 25 minutes until the surface springs back when touched and a toothpick inserted in center comes out clean. Let cool in pan for 10 minutes. Then while wearing oven mitts, push base of tin upwards from bottom to release tart from sides of pan. You may want to use a spatula to also slide the tart off the base onto your serving plate; however, if it starts to break apart, the nearly invisible base of the flan pan enables you to use it for service with little consequence to your presentation.