In a large mixing bowl, cream together the butter and sugar until light and fluffy.

Add the eggs, one at a time, beating in until fully incorporated.

Spoon in the vanilla and beat in.

If adding the lime zest, do so now. Beat in until fully incorporated.

Sieve in the flour and mix in until no flour is visible. Avoid over mixing here.

Split the cake mix evenly between your two tins and bake on the middle shelf for 35 - 40 minutes. This timing will depend on how hot your oven runs. Check after 35 minutes as they are pretty thick sponges!

Your cake sponges will be ready when the surfaces bounce back when gently pressed on; a skewer comes out clean when tested, and you hear little to no bubbling/crackling when you listen to them.

Once baked, allow to cool in the tins for around 10 minutes before transferring upside down to a wire rack to cool completely.

While your sponges are cooling, make your Swiss meringue buttercream.

Clean the bowls, whisk attachment and beaters and any spoons you may use with the lemon juice or white wine vinegar. This makes sure that there is no grease that can prevent the egg whites reaching stiff peaks.

Separate each egg, putting the yolks in one bowl and the whites in a small bowl, then into a large bowl. Good advice would be to pour the egg white into the large bowl every time you separate an egg, so that you don’t risk ruining the whole batch of egg whites if any yolk gets in.

Whisk the sugar into the egg whites briefly by hand. Save the yolks for another meal.

Set the bowl over a bain marie (saucepan of hot simmering water). Make sure the bowl does not touch the water.

Whisk the sugar and whites by hand over the heat until the mixture is thinner and the sugar has completely dissolved. It’ll be tacky to begin with but will become lovely and frothy when it’s ready.

To test that the mixture is heated through and the sugar has dissolved, quickly dip your finger lightly into the mixture and rub it between your finger and thumb. If you can still feel sugar granules, it needs longer. If the sugar granules are non-existent, it’s ready. Be careful doing this as the mixture will be hot!

If wanting to use a food grade thermometer, leave it in during this process until the mixture has reached 71°C [160°F].

Take the bowl from the heat and prepare a standing or electric hand mixer with a whisk attachment. You don’t need to wait for the mixture to cool as it’s better for the mixture to be warm for this stage.

On a medium-high speed, whisk the mixture to stiff peaks. They should be glossy, too! Depending on the weather and the temperature of your kitchen, this can take 10+ minutes. You want the mixture to be cool by the end.

If it’s not reaching stiff peaks, place in the fridge uncovered for around 10 minutes before trying again.

Make sure that the meringue is fully cool before moving onto the next step.

Change the whisk attachment out for a paddle or set of beaters.

Add the butter, one block at a time, and beat in until each cube is fully incorporated.

Once all the butter has been added, add the vanilla and the salt and beat in briefly. See the Top Tips section in the blog post for troubleshooting tips.

Once your cakes are cool and your Swiss meringue buttercream is ready to use, begin building your cake.

Smear a little buttercream onto a rotating cake stand/plate/cake board. Place the first sponge on top of this.

Smother it in a generous layer of your Swiss meringue buttercream.

Place the second sponge on the top. Cover the rest of your cake with Swiss meringue buttercream - just be sure to save some for the piped decoration.

Using a ridged cake smoother or a standard culinary fork, gently push the ridges into the sides of your cake and turn it, holding your smoother or fork steady and moving up after each layer is done. This creates a striped pattern similar to that on the cake in the film

None of your cake decoration needs to look perfect or overly neat. We are aiming for the cake to look like it would after Robin Williams has pushed his face into it, so it shouldn’t be as neat as a pin!

Using a palette knife, mess up the buttercream on the top of the cake to make it look like a face. Keep it simple but make it look as messy as you want! Alternatively, you can wait to actually push someone’s face into it. Your call.

Spoon the remaining Swiss meringue buttercream into a piping bag or sandwich bag with an open star piping tip attached. Pipe a swirling or folding pattern around the top of the cake. This part is up to interpretation as we never actually see the top of the cake in the film.