• Preheat the oven to 180C/350F.

Making the biscuit base

• Add the biscuits into the bowl of a food processor bowl and whizz until you get fine crumbs. Alternatively, throw the biscuits into a sturdy plastic bag and give them a good bashing with a rolling pin.
  250 grams (9 oz) digestives biscuits/graham crackers
• Melt the butter in the microwave or a small saucepan on the stove. Pour it over the biscuit crumbs and mix until well combined, ensuring all the crumbs are coated with butter.
  100 grams (1 US stick) unsalted butter, melted
• Transfer the crumb mixture into a tart tin or pie dish, approximately 23cm/9" in diameter.
• Press the crumbs firmly into the dish, ensuring they cover the bottom and reach up the sides of the tin. Use the back of a spoon or the bottom of a glass to compact the crumbs firmly in the dish.
• Place the prepared tart base in the fridge until needed.

Making the lemon filling

• Separate the eggs, being careful not to let any traces of egg yolk into the egg whites.
• In a large bowl, combine the egg yolks, condensed milk, lemon juice and lemon zest.
  4 egg yolks, 400 grams (14 oz) condensed milk, 120 millilitre (½ US cup) fresh lemon juice, zest from one lemon
• Whisk the mixture thoroughly using an electric hand mixer or a hand whisk until fully incorporated, approximately 1-2 minutes. The condensed milk will slightly thicken due to the lemon juice.
• Gently pour the lemon filling into the tart base and tilt the tin from side to side to level it.
• Bake the pie in the preheated oven for 20-25 minutes, or until the sides are firm and there is only a very slight wobble in the middle.

  Be careful not to underbake, as the tart may not set firm enough.
• Let the pie cool, then transfer it to the fridge to fully set and chill for at least 4-6 hours, or overnight.

Making the Swiss meringue

• Attach the whisk attachment to a stand mixer or an electric hand mixer.
• Fill a saucepan with at least 5cm/2" of boiling water. Place a bowl on top of the saucepan, ensuring the bottom of the bowl does not touch the water. Alternatively, if you have a double boiler, use that.
• Add the caster sugar and egg whites to the bowl. Make sure you have a hand whisk ready.
  4 egg whites, 220 grams (8 oz) white caster sugar
• Over medium heat, whisk the egg white and sugar mixture continuously until all the sugar has melted. Expect the mixture to become very foamy as it heats up.
• To check if the sugar has melted, quickly dip your fingertips into the mixture and feel for any graininess. Alternatively, if you have an instant-read thermometer, the temperature should read around 71C/160F.

  Typically, this process takes around 5 minutes.
• Transfer the hot egg and sugar mixture to the grease-free bowl of a stand mixer. If you're using an electric hand mixer, transfer the mixture to a large, grease-free bowl.
• Beat the meringue mixture on medium-high speed for 1-1½ minutes until it starts to build a little volume. Then, add the cream of tartar, vanilla extract and a generous pinch of salt.
  ½ teaspoon cream of tartar, 1 teaspoon vanilla extract, ¼ teaspoon salt
• Continue beating until the meringue turns pure white with stiff, glossy peaks that hold their shape. Stop as soon as you reach this stage as overbeating might cause graininess.

  It should take approximately 3-4 minutes with a stand mixer on medium-high speed and a little longer with an electric hand mixer.
• If the meringue still feels a little warm, let it cool on its own for about 10 minutes. Spoon generous, billowing clouds of meringue on top of the lemon filling, or use a piping bag.
• Use a kitchen torch to caramelise the tips and bulges of the meringue or briefly place the pie under the broiler in the oven, making sure it's not too close to the element. Keep a very watchful eye on it!