01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a small bowl, combine ground flaxseed and water. Set aside for 5 minutes to thicken into a flax egg.
03 - In a large mixing bowl, whisk together melted coconut oil and sugar until smooth. Add pineapple juice, vanilla extract, and the prepared flax egg; mix until well blended.
04 - In a separate bowl, whisk together all-purpose flour, baking soda, baking powder, and fine sea salt until evenly distributed.
05 - Add dry ingredients to the wet mixture, stirring until just combined. Gently fold in diced pineapple and plums, being careful not to overwork the dough.
06 - Scoop tablespoon-sized portions of dough and roll into balls. Roll each ball in the extra granulated sugar to coat evenly.
07 - Place dough balls on prepared baking sheets, spacing 2 inches apart. Gently flatten tops with your fingers or the bottom of a glass.
08 - Bake for 10 to 12 minutes, until edges are just turning golden and centers look set.
09 - Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.