Cream the butter and sugar together with an electric mixer, starting with slow speed and gradually increase to medium speed, 5-7 minutes or until light and fluffy, scraping down the sides occasionally. Reduce the speed to low and mix in the egg and the flour just until combined. Turn the dough out onto a clean, oil-free and flour-free work table. If the dough is crumbly, just gather them together until they form a dough.
Press dough into the tart shells (thickness around 2-3 mm) and place on baking sheets. The dough will puff a little during baking.
Half-bake the shells on the medium rack for 15 to 18 minutes for smaller tarts and 20 to 22 minutes for the larger ones until they are slightly browned. Remove from the oven and set the baking sheets onto racks to cool completely.
Filling
Cream the butter and sugar together with an electric mixer for 5-7 minutes. Reduce speed to low and mix in the eggs, egg yolk, ground almond, flour and vanilla essence.
Assembly and Baking
Reduce the heat of the oven to 160 degrees Celsius.
Pipe (or spoon) the filling into pastry shells and bake on the lower rack until the crust is golden brown while the filling is golden, 20 minutes for the smaller tarts and 24 minutes for the larger tarts.
Transfer the tarts onto racks to cool slightly. When cool enough to handle, unmould the tarts out and let cool completely on a wire rack.