#define Btn1_GPIO 35
#define LED1_GPIO 5

void IRAM_ATTR Ext_INT1_ISR() {
  digitalWrite(LED1_GPIO, !digitalRead(LED1_GPIO)); 
}

void setup() {
  pinMode(LED1_GPIO, OUTPUT);
  pinMode(Btn1_GPIO, INPUT);
  attachInterrupt(digitalPinToInterrupt(Btn1_GPIO), Ext_INT1_ISR, RISING); 
}

void loop() {
  // No need for code in loop() for this simple interrupt example
}