#define LED_PIN     21
#define BUTTON_PIN  4

void IRAM_ATTR toggleLED() {
  digitalWrite(LED_PIN, !digitalRead(LED_PIN));
}

void setup() {
  Serial.begin(115200);
  
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT);
  
  attachInterrupt(BUTTON_PIN, toggleLED, FALLING);
}

void loop() {
  delay(10); // this speeds up the simulation
}