Build base: Melt 1–2 cubes of butter in a small saucepan over low heat. Add the garlic and cook, stirring, until fragrant and just softened, about 1–2 minutes. Don't let it brown.
Simmer: Pour in ¼ cup of water and whisk to combine. Bring to a light simmer — you're looking for small, steady bubbles around the edges, not a rolling boil.
Emulsify: Reduce heat to the lowest setting. Add the remaining cold butter one cube at a time, whisking constantly. Wait until each cube is fully melted and incorporated before adding the next. The sauce should look smooth and slightly creamy as it comes together.
Finish: Remove from heat. Stir in the parsley and season with salt and pepper to taste.