Preheat the oven to 180 degrees Celsius or 350 Fahrenheit. Grease or line a 27cm rectangular slice tin.
In a food processor, combine the butter, sugar, salt, flour and baking powder and pulse until it resembles coarse bread crumbs. Alternately use a pastry cutter or your fingers to cut/rub the butter into the flour.
Lightly beat the egg and add it to the flour mixture. Stir or pulse it to combine it into a thick dough that clumps together (not sticky), along with as much of the milk as you need to make the dough.
Scoop out the inners of the feijoas and place them in a bowl. Add in the cinnamon and cornflour and use a fork to mix this together and slightly squash the feijoas and break them up a bit.
Take 2/3 of the dough mixture and press it into the slice tin. Use the back of a measuring cup to press it down until the bottom of the tin is covered.
Layer over the feijoa filling.
Take the remaining 1/3 of the dough and rip off pieces and flatten them slightly. Place the pieces irregularly over the feijoa filling. It is fine that it doesn't cover all the filling.
Bake for 25-28 minutes until the shortcake topping is golden brown. Leave to cool for at least 10 minutes before slicing.