Prep the fat: Gently melt your fat until fluid, then let it cool to just warm or room temp. Hot fat can break the emulsion.
Add base ingredients to the jar: Crack the egg into the jar. Add vinegar and salt (if using).
Pour in the fat: Add all the melted-and-cooled fat on top of the egg mixture.
Blend from the bottom: Place the immersion blender at the bottom of the jar. Turn it on and hold it steady for 10–15 seconds. You’ll see a thick, pale emulsion form at the base.
Lift slowly: Once the bottom thickens, slowly raise the blender head, letting it pull the remaining fat down and emulsify. This takes another 10–20 seconds.
Adjust thickness: If it looks too thick, blend in 1–2 teaspoons warm water. If too thin, drizzle in 1–2 tablespoons more melted-but-cool fat while blending.
Taste and tweak: Add a tiny splash more acid or a pinch of salt if needed. Blend just to combine.
