To make the biscuit dough, put the flour, custard powder, and baking powder into the bowl of a stand mixer and combine with a paddle attachment. Next, add 3 tablespoons butter, and vegetable shortening and combine until the mixture is crumbly.

Add the sugar and mix again. Next, beat the egg and milk together, and add to the stand mixture and combine until it forms a dough ball. Be careful not to over mix. Wrap dough in plastic wrap, and let rest in the fridge for 20 min.

Roll dough out on lightly floured surface to a thickness of about 4 mm. Cut out with a small heart cookie cutter. Use a toothpick to prick the outside edge of the heart to create the small hole pattern for half of the biscuits, to use as the tops of the sandwich cookies.
Bake on baking sheets at 350, for 10-14 min. until lightly browned at the edge. Let cool completely on a wire rack before filling.

To make the custard cream filling, put icing sugar, and custard powder, and butter in the bowl of a stand mixer and combine slowly at first and then faster until the mixture is fully combined. Add the hot water and fully combine until smooth, using a silicone spatula to scrape down the sides of the bowl.

To assemble the cookies, spread one half of a cookie with the custard cream filling using a palette knife, and sandwich with the top of cookie. Store in an airtight container.