#define LED 12
#define BTN 4

// ms delay to avoid bouncing problems
#define DEBOUNCE_DELAY 50

// last time the button has been triggered
volatile int last_millis = 0;

// interrupt function
void IRAM_ATTR trigger_led() {
  if(millis() - last_millis > DEBOUNCE_DELAY)
    digitalWrite(LED, !digitalRead(BTN));
  
  last_millis = millis();
}

void setup() {
  pinMode(LED, OUTPUT);
  pinMode(BTN, INPUT_PULLUP);
  
  attachInterrupt(BTN, trigger_led, RISING);

  Serial.begin(115200);
}

void loop() {
  // the loop part can be empty
  // because all the logic is "event-based" thanks
  // to the interrupt
}