The answer, according to multiple sources, is with Beuti's Beauty Sleep Elixir, an overnight face oil that is the signature and only product of the independent UK based company.
Beuti Skincare's founder Leila Aalam even claims that Kate keeps three of the bottles on her bedside table.
“I wanted to make a product that truly makes your skin glow and that was as organic as possible,” she says in an interview with People magazine.
“We were ecstatic to know that [the Duchess of Cambridge] liked it and was actually using it. Kate has got great skin, but as she gets older she will need to use more targeted skin care – plus, having children makes your skin more sensitive.”
The oil is 100% natural and 98% organic, boasting nutrient dense ingredients and 14 natural oils.
Aalam, a trained beauty therapist, also included Caribbean coral extract in the formula: a powerful anti-inflammatory that helps to neutralise one of the enzymes responsible for irritating and inflaming the skin, giving the oil it's anti-ageing properties.
According to Aalam, the Beauty Elixir was given to Kate directly through a friend.
If you have no such friend that delivers miraculous, glow-enhancing gifts then the highly sought oil can be purchased online for $AUS72.