#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(Btn1_GPIO, Ext_INT1_ISR, RISING);
}

void loop() 
{
  // Do nothing... 
}