"天鹅绒般柔软的" can be translated as "velvety soft" or simply "velvety" in English, depending on the context and the desired level of detail.
"Velvety soft" emphasizes both the velvety texture and the softness.
"Velvety" alone also conveys the idea of a soft, smooth texture similar to velvet.
For example:
The fabric feels velvety soft against the skin. （这块布料贴在皮肤上感觉天鹅绒般柔软。）
The cake has a velvety texture. （这个蛋糕有天鹅绒般的质地。）