The Impossible Cookie is a fan-made modification of Cookie Clicker version 2.021; the game itself is Orteil's, and the mod's own readme credits him. It sets one goal: bake 1,000,000,000 cookies. The catch is that every whole cookie you earn drops a real, physical cookie onto the screen. They fall from the top, bounce off each other and the walls, and pile up at the bottom, driven by the matter.js physics engine. None of them ever leave.
Each falling cookie is a physics body the browser has to simulate on every frame. A few hundred are fine. Many thousands slow everything down, and a billion is far beyond what any computer can keep moving. The faster your cookies per second climb, the faster the pile grows, so the game works against its own progress. The mod's author adds an extra challenge: reach the goal without reloading the page once.
Underneath the falling cookies it plays like Cookie Clicker 2.021. Click the big cookie, buy buildings in the store (seventeen of them, from the Cursor to the Javascript console), pick up upgrades and click golden cookies when they appear. The Garden, Grimoire and Pantheon minigames are here; the Stock Market is not, because it arrived in a later version. You can grab a fallen cookie with the mouse and drag it around the screen.
Your bakery is saved in your browser's local storage, so a reload keeps your cookies and buildings. The physical cookie pile is not saved: reloading clears the screen, which is also the one way to make a slow game smooth again, if you do not mind breaking the no-reload challenge.