Start the Pastry Cream: Pour the milk into a medium saucepan and heat it over medium heat until it's steaming and small bubbles form around the edge. Don't let it boil.

Whisk the Base: In a separate medium bowl, whisk together the caster sugar, cornflour, and egg yolks until the mixture is pale, smooth, and lump-free.

Temper the Eggs: Slowly and carefully, pour about half of the hot milk into the egg yolk mixture while whisking constantly. This is called tempering and it prevents the eggs from scrambling. Pour the tempered egg mixture back into the saucepan with the remaining milk.

Thicken the Cream: Return the saucepan to medium heat. Whisk continuously until the cream begins to thicken and bubble, which should take about 3-5 minutes. Once it starts bubbling, keep whisking for one full minute to cook out the cornflour taste. For a detailed guide on this technique, BBC Good Food has a great visual on making crème pâtissière.

Finish and Chill the Cream: Remove the pan from the heat and stir in the cold butter cubes and vanilla bean paste until fully incorporated. Pour the pastry cream into a clean bowl and press a piece of cling film directly onto the surface to prevent a skin from forming. Refrigerate for at least 1 hour, or until completely cold and set.

Make the Cookie Dough: In a large bowl using a stand mixer or hand mixer, cream the softened butter with the light brown and caster sugars until light and fluffy, about 3-4 minutes. Beat in the egg and vanilla extract, followed by the mashed banana.

Combine and Chill Dough: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix. Cover the bowl and chill the dough for at least 30 minutes. I find that what works best for me is a full hour of chilling; it really helps the cookies hold their shape.

Bake the Cookies: Preheat your oven to 180°C (160°C fan) and line two baking trays with parchment paper. Scoop rounded tablespoons of dough onto the prepared trays, leaving about 5cm of space between them. Bake for 12-14 minutes, or until the edges are golden brown.

Create the Well: As soon as the cookies come out of the oven, use the back of a teaspoon or a small round object to gently press an indentation into the centre of each warm cookie. Let them cool on the tray for 5 minutes before transferring to a wire rack to cool completely.

Assemble Your Cookies: Once the cookies are completely cool, give the chilled pastry cream a good stir to loosen it up. Spoon or pipe the cream into the well of each cookie. Sprinkle the crushed vanilla wafers over the cream just before serving.