01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large mixing bowl, beat the softened vegan butter and granulated sugar together using an electric mixer or whisk until pale, light, and fluffy. Pour in the plant milk and vanilla extract, then mix until fully incorporated.
03 - In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until evenly distributed.
04 - Gradually add the dry ingredient mixture to the wet ingredients, stirring gently with a spatula or wooden spoon until the dough just comes together. Avoid overmixing to keep the cookies tender.
05 - Scoop tablespoon-sized portions of dough and roll each into a smooth ball. Arrange on the prepared baking sheets, spacing them about 2 inches apart. Gently press each ball down with the palm of your hand to flatten slightly.
06 - Bake for 10 to 12 minutes, or until the edges are lightly golden. Allow the cookies to rest on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
07 - While the cookies cool, combine the diced pineapple, fresh blackberries, lemon juice, and 2 tablespoons of granulated sugar in a blender or food processor. Blend until completely smooth, then strain the purée through a fine mesh sieve to remove seeds and excess pulp.
08 - In a medium bowl, combine the powdered sugar and softened vegan butter. Add 2 to 3 tablespoons of the strained fruit purée along with 1 to 2 tablespoons of plant milk, adjusting as needed. Whisk vigorously until the filling is smooth, creamy, and easily spreadable.
09 - Once the cookies are completely cool, spread a generous layer of the pineapple blackberry cream on the flat side of half the cookies. Place a second cookie on top of each frosted half, flat side down, and gently press to form a sandwich.