1) Wash and dry 4 large pink or ruby grapefruits. Zest two grapefruits using a fine grater or microplane, avoiding the bitter white pith. Cut all 4 in half and juice them; you should get about 2 cups (≈480 mL) of juice. Strain the juice through a fine mesh to remove pulp and seeds.

2) In a small saucepan combine 1 cup (240 mL) water and 2 cups (400 g) granulated sugar. Add the grapefruit zest. Warm over medium heat, stirring until the sugar is fully dissolved. Bring just to a simmer for 2–3 minutes to extract the oils from the zest; do not boil vigorously.

3) Remove the pan from heat and let the syrup steep with the zest for 10–15 minutes off the heat for more aroma.

4) Strain the hot syrup through a fine mesh to remove zest. While the syrup is still warm, stir in the strained grapefruit juice, 1 tablespoon fresh lemon juice (optional, for brightness), and 1/4 teaspoon citric acid or 1/4 teaspoon kosher salt (optional, for stability and balance). Taste and adjust: add up to 1–2 tablespoons extra lemon juice if you prefer more acidity.

5) Cool the cordial to room temperature, then transfer to a clean, sterilized glass bottle or jar. Seal tightly and refrigerate.

6) Label with date. Shake before each use; the cordial may separate slightly.

7) Use in cocktails (substitute for grapefruit liqueur/juice), mix with soda water for a soft drink, or drizzle over desserts. Freeze in ice cube trays for long-term storage.

8) Discard if you notice any off smells, visible mold, persistent cloudiness change, or fermentation (bubbling).