For the Canary Cream biscuit cookies:

1. Line 2 baker's half sheets with parchment paper and reserve to the side.
2. In a large mixing bowl, whisk flour, custard powder, baking soda, and salt.
3. Add the cold butter (grated or finely diced) and work in with your fingertips until the mixture is gravelly and resembles breadcrumbs.
4. Add the caster sugar, egg yolk, milk, and vanilla extract and stir with a wooden spoon to combine. I find that once the mixture is semi-combined, it works best to combine the rest by hand, working the mixture until it is a smooth dough. At first, it will seem like you need more liquid, but don't be tempted to add any. As you keep working the dough, it should come together.
5. Lightly flour your rolling surface and your rolling pin, then roll the dough out to approximately 3-4 mm (about 1/4 inch or just under).
6. Using the bird biscuit/cookie cutter, cut the first 12 cookie biscuits out of the rolled dough (You may need to roll the dough more than once to do this). Transfer the cookies to the first sheet pan. Using the 1 1/2-inch round biscuit cutter, cut a hole in the "belly" of each bird. Remove the circle of dough from each of the birds "bellies" and return it to the remaining dough on your counter to use for the other half of the cookie biscuits.
7. You will need to roll, cut, and reroll the dough several times, but should end up with an additional 12 bird-shaped cookie biscuits (24 total cut cookie biscuits from the dough). Place the remaining 12 cookie 'bottoms' "face-side-down" on the second tray. This helps create a uniform look to the biscuit cookies (**see notes below). Do not cut a circle in these.
8. Transfer both baking sheets to the refrigerator for 30-minutes. This will help the cookie biscuits hold their shape when baking. Preheat your oven to 350 degrees F / 175 degrees C so that it comes to temp while the cookie biscuits chill.
9. Transfer the chilled cookie biscuits to the oven and bake for 12-16 minutes or until they are just beginning to turn golden brown at the edges. Allow the biscuit cookies to cool fully before removing them from the baking sheets.

For the vanilla custard cream filling and to assemble:

1. In a small mixing bowl, combine the softened butter, sifted powdered sugar, custard powder, vanilla, milk, and salt, whisking until smooth, pale, and creamy.
2. Flip the cooled Canary Cream cookie biscuit bottoms over so that the side touching the pan is exposed. The bottoms and the tops should all now be facing the same direction.
3. Fill a piping bag with the vanilla custard cream and snip a small hole in the end. You don't need to do any fancy piping but I like to use a piping bag because it helps me fill the irregular shape of the birds easier. Pipe filling into each bird 'bottom' half then place the bird 'top' halves (with the circles cut out) on top of each, gently sandwiching each cookie.
4. While the filling is still fresh, top each bird 'belly' with a sprinkle of the yellow sanding sugar.
5. Store leftover Canary Cream biscuit cookies in an airtight container at room temperature or in the fridge for up to 3 days.