I tried your code, but changed the buffer size, since I only need to display 1 character 0-9. Did I do it right? I get an error:
invalid conversion from 'char*' to 'unsigned char' [-fpermissive]
int theInt = 6; //integer to be converted
char buffer[2]; //char buffer to hold converted integer up to 9 digits plus terminator
itoa(theInt, buffer, 2); //convert using base 10 and put result in string named buffer
tft.drawChar( 190, 100, buffer, ILI9341_WHITE, ILI9341_BLACK, 5);