Preheat oven to 210°C/190°C fan/Gas Mark 7.
Line a 40 x 28 swiss roll tin with greaseproof paper.
In a stand mixer, whisk together the eggs, sugar, warm water and vanilla and until the mixture is very light and fluffy and you can drizzle a ribbon with the mixture before it disappears.
Once it’s ready, carefully sift over the plain flour and fold in gently with a large metal spoon until completely combined.
Pour into the tin and use a spatula to spread evenly. Bake for 12-15 minutes.
Meanwhile dissolve the milkshake powder in 4 tbsp of milk to form a paste, mix well until smooth and set aside.
While the cake is baking, lay out a large teatowel and sift a thin layer of icing sugar. Once the cake is baked, turn it immediately onto the tea towel and peel off the greaseproof paper. Roll up with the tea towel, starting from the short end so you have a short, thick roll. Allow to cool completely in the tea towel roll.
While the sponge is cooling, beat the butter and icing sugar together until smooth. Remove 120g and loosen with 1-2 tsp of milk to make a piping consistency. Transfer to a piping bag fitted with a large star nozzle.
Add the milkshake paste to the remaining buttercream and a little pink food colouring to make a pink ‘strawberry milkshake’ colour. Set aside until needed.
Once the sponge is cool, unroll completely and slice it into three long, even strips. Spread each with 4 tbsp of the pink buttercream mixture.
Starting with the short end of one strip, roll it up. Once the strip is rolled, position the exposed end at the beginning of the next strip and keep rolling. Again, once this is rolled, position the exposed end at the beginning of the last strip and continue to roll. You now have a rolled cylindrical cake. Wrap in cling film and freeze for 5 minutes.
Once the cake has chilled, stand upright so you can see the swirl on the top and then cover the cake with the pink buttercream, smoothing over the top and sides. Once smooth, use the white buttercream to pipe 6 rosettes around the top edge and top each with a halved strawberry.