marzipan layer
n. In a business or professional firm, the level of managers and other senior staff just below the topmost level of directors or partners.
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A few FTSE 100 companies have three or more women directors, including Marks and Spencer, Legal & General and J Sainsbury.

Cranfield is looking at what is known as the "marzipan layer" of talented people just below board level, whom Higgs argues should be encouraged to gain nonexecutive experience on noncompetitor boards.

Vinnicombe says the marzipan is uneven; some companies have no women at this level either.
—Alison Maitland, “UK boards reluctant to bet on 'outsiders',” Business Day (South Africa), November 04, 2003
A recent report by Laura Tyson, head of the London Business School and a former chairman of America's Council of Economic Advisers, notes that 30% of British managers are female. But many are in the "marzipan" layer just below the top-executive icing, from which non-executive directors are rarely picked.
—“Be a man,” The Economist, June 28, 2003
1985 (earliest)
The marzipan layer is on the move.

Every few days now there is news of a further shift of some team of City brokers dealers from one house to another, mostly of people just below the partner level, hence the expression. The partners, typically pocketing between pounds 500,000 and pounds 2 million for their share of the cake, are the icing. Those just below, who have to rub along on a relatively meagre income of less than pounds 100,000 a year, are less than pleased at seeing their elders' behaviour. Hence the urge to move.
—Hamish McRae, “Financial Notebook,” The Guardian, February 14, 1985
Some cakes have a layer of marzipan — a paste made from almonds, sugar, and egg whites. This is usually found just below the icing, hence it servies as the underlying (so to speak) metaphor for the head-scratchingly common business phrase, marzipan layer.