Cooktop appliance and griddle assembly

A cooktop appliance defining a vertical direction, the cooktop appliance comprising:

• a top panel;
• a heating element attached to the top panel;
• an upper plate selectively disposed above the top panel along the vertical direction, the upper plate having a top cooking surface and a bottom heating surface, the top cooking surface extending perpendicular to the vertical direction to receive a cooking item thereon, the bottom heating surface positioned beneath the upper plate and facing the top panel to receive a thermal output from the heating element;
• a lower plate fixed to the upper plate and extending perpendicular to the vertical direction beneath a portion of the upper plate and above the top panel, a radiation channel being defined between the lower plate and the bottom heating surface along the vertical direction;
• and
• a perimeter rim extending in the vertical direction from the upper plate to a position below the lower plate, the perimeter rim defining an exhaust cavity above the heating element, wherein the exhaust cavity is in fluid communication with the radiation channel and the heating element.