26 USC 6671 - Rules for application of assessable penalties

(a) Penalty assessed as tax 
The penalties and liabilities provided by this subchapter shall be paid upon notice and demand by the Secretary, and shall be assessed and collected in the same manner as taxes. Except as otherwise provided, any reference in this title to tax imposed by this title shall be deemed also to refer to the penalties and liabilities provided by this subchapter.
(b) Person defined 
The term person, as used in this subchapter, includes an officer or employee of a corporation, or a member or employee of a partnership, who as such officer, employee, or member is under a duty to perform the act in respect of which the violation occurs.