A cooktop appliance includes a gas burner. A primary control valve is coupled to the gas burner. The primary control valve is manually adjustable to regulate a flow of gaseous fuel to the gas burner. A secondary control valve is connected in series between the primary control valve and the gas burner. A controller is in communication with the secondary control valve. The controller is configured to receive a temperature measurement from a temperature sensor and to adjust the secondary control valve in response to the temperature measurement from the temperature sensor.