Originated by Danny Van Eechaute of Belgium, this non-registered cultivar is a miniature size (6 inches tall by 14 inches wide) sport of H. 'Mouse Party'. The foliage is thick, rounded and blue-green with a yellow marginal variegation. It is distinguished by having a narrow white "flash" of color where the yellow meets the blue-green. Lavender flowers are produced on short, stout scapes in July.

According to the In the Country Garden & Gifts website, "'Cool Mouse' is a good grower, but a little difficult to produce in large numbers."