Whisk the dry ingredients: In a medium bowl, whisk together the flour, baking powder, bicarbonate of soda, and salt. Set this aside. The flour should feel light and airy, with no lumps of powder visible.

Cream the butter and sugar: In a large bowl, beat the softened butter and caster sugar with an electric mixer on medium speed for about 3 minutes, until the mixture turns pale and fluffy. It should look like a thick, sandy paste and smell sweet and buttery.

Add the wet ingredients: Beat in the egg yolks one at a time, then add the vanilla extract, lemon zest, and lemon juice. The mixture will look slightly curdled at this point—that's completely normal and actually helps create the soft texture. You'll smell the fresh lemon immediately.

Combine wet and dry: Gradually add the flour mixture to the wet ingredients, mixing on low speed until just combined. The dough will be soft, a little sticky, and very fragrant. Scrape down the sides of the bowl with a spatula.

Chill the dough: Cover the bowl with cling film and refrigerate for at least 1 hour, or up to 3 hours. The dough needs to be firm enough to roll into balls without sticking to your fingers. This is the perfect moment to put on some music and tidy the kitchen.

Preheat and prep: When you're ready to bake, preheat your oven to 180°C (160°C fan). Line two baking trays with parchment paper. Place the icing sugar in a shallow bowl.

Roll and coat: Scoop out rounded tablespoons of the chilled dough (about 25g each) and roll them into smooth balls. Roll each ball generously in the icing sugar until completely coated. The sugar coating is what creates the beautiful crackled, crinkled top as they bake.

Bake: Place the sugar-coated dough balls on the prepared trays, leaving about 5cm between each one. Bake for 10-12 minutes. The cookies will look puffed and cracked, with edges that are just beginning to turn a pale gold. The centres will still look soft and slightly underdone—that's exactly what you want.

Cool: Let the cookies rest on the baking tray for 5 minutes. During this time, they will continue to set and the crinkles will deepen. You'll see the beautiful, snowy cracks forming. Then transfer them to a wire cooling rack to cool completely.