She cemented her superstar status when she created YSL's iconic Touche Éclat concealer (you know, the magic pen sold every 10 seconds somewhere in the world), before going on to found her own cosmetics company, the beauty editor and celebrity fave By Terry. But the seriously accomplished make-up artist's latest creation could be her best yet - a reimagined Light Expert Foundation Click Brush (launching in Mecca today). If it looks familiar, that's because it is - it's an update on de Gunzburg's original By Terry foundation brush (the first of its kind on the market, one of which sold every two minutes when it first launched in 2007), but with a better brush (an angled Kabuki for flawless application) and high-tech formulation that allows for all-day touch-ups, even over make-up set with powder.

What's de Gunzburg's secret? Her success could have something to do with her in-depth knowledge of the chemical processes behind her products - she did study medicine before moving into make-up after all. Or it could be her tutelage under the master of colour Yves Saint Laurent himself, who she worked with as creative director of YSL make-up from 1985 until 1998, when she left to launch her own line. Below, the make-up legend shares her beauty secrets, from why you should use a different foundation at night to French women's skincare essentials.

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Beauty secrets from the creator of YSL's Touche Eclat and By Terry

What's your secret to achieving the perfect foundation application?
Starting with a CC (colour corrector) primer is a great way to brighten, reduce dark spots and take out any redness of the complexion in order to prepare an even base.  Always use a foundation brush instead of your fingers or a sponge, it makes colour and coverage longer lasting, uses less product and can reach the smallest areas and creases that fingers or a sponge won't. My By Terry Light Expert is perfect for this!

Can you share any tricks for getting the colour right?
I prefer to wear a cool-toned foundation shade for evening make-up (a lighter tone has anti-fatigue benefits) and warm up the tone (but not too dark) for day time as it gives a healthy natural glow. 

What products could you never live without?
My kit consists of Baume de Rose for plump and hydrated lips, Light Expert Click Brush for foundation and touch-ups, Mascara Terrybly for separated long lashes with the extra benefit of hydration and a growth-boosting effect and eyebrow mascara for defined brows. I use Terrybly Densiliss Concealer to lighten the eyes, Terrybly Velvet Rouge in INGU RED for chic glamour and Hyaluronic Hydra Powder to fix it all!

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Beauty secrets from the creator of YSL's Touche Eclat and By Terry

Why did you choose make-up over medicine?
I chose make-up over medicine because I have a natural inclination to aesthetics and art.  I wanted to be creative and to recreate beautiful colours and textures. My medical studies, however, gave me the understanding of how things are made. Experimenting for new cosmetics is extremely scientific, especially if the brand is like By Terry and aiming to research the newest groundbreaking technologies. Thanks to my scientific studies I have an understanding of this. 

What's been your career highlight?
Of course [creating] Touché éclat when I was make-up creative director at YSL and creating my own brand, but also the creation of Baume de Rose, my number one bestseller worldwide - this was the result of an error in the lab! Now this cult product is the most loved by celebrities and every day women!

Related story: Are you wearing too much blush? 

Beauty secrets from the creator of YSL's Touche Eclat and By Terry

What defines the French woman's approach to beauty?
Simplicity, quality, effortlessness!

What can the rest of the world learn from French women when it comes to skincare and make-up?
To only use the best quality products and to invest in your make-up and skincare. To always protect your face body and hands in the sun!  And to never underestimate red lipstick!

Beauty secrets from the creator of YSL's Touche Eclat and By Terry

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