Preheat your oven to 180°C (350°F). Line two baking trays with parchment paper or silicone baking mats. This ensures even browning and prevents sticking. Having everything ready before you start mixing means you can go straight from dough to baking without delay.

Add the softened butter and caster sugar to your Thermomix bowl and mix on Speed 5 for 1 minute. This creaming process incorporates air into the butter, which helps create a lighter, more tender cookie texture. The mixture should look pale and slightly fluffy when done.

Pour in the sweetened condensed milk and vanilla extract, then mix on Speed 5 for 30 seconds. The condensed milk adds richness and sweetness while reducing the need for extra sugar, and it keeps the cookies wonderfully soft. Mix just until combined—overmixing at this stage isn't necessary.

Sift your self-raising flour with the fine sea salt, then add it to the Thermomix in three equal portions, mixing on Speed 6 for 20 seconds after each addition. I find adding flour in parts prevents overmixing and helps distribute it evenly without developing too much gluten, which keeps cookies tender rather than tough. After the final addition, the dough should come together in a soft, slightly sticky ball.

Transfer the dough from Step 4 to a work surface lightly dusted with flour. Roll the dough into balls about the size of walnuts, then place them on your prepared trays, spacing them about 2 inches apart. Gently flatten each ball with the back of a fork, pressing down lightly to create a slight indent—this helps them cook evenly and gives them that classic cookie texture.

Bake on the middle shelf of your preheated oven for 10-12 minutes, until the edges are just lightly golden but the centers still look slightly underdone—I like to remove them when they still have a tiny bit of softness in the middle because they continue to cook as they cool on the tray. Remove from the oven and let the cookies rest on the baking trays for 10 minutes before transferring them to a wire rack to cool completely. This resting period is crucial because it allows the centers to set without the cookies becoming hard.