Start by heating the fresh raspberries with lemon juice or water in a small saucepan over medium heat. Mash the berries as they warm up until they reach a saucy consistency. Let the mixture cool slightly, then stir in chia seeds and transfer to a jar to chill for at least an hour.
Preheat the oven to 175°C (350°F). In a food processor, pulse together flour, almond meal, cornstarch, and salt. Add cold butter or coconut oil in cubes, pulsing until it resembles coarse crumbs. Slowly add almond milk and vanilla, pulsing until a dough ball forms.
Divide dough into two disks and roll one out on a floured surface to about 1/4-inch thickness. Use heart-shaped cookie cutters to cut out shapes. Carefully place on a baking sheet lined with parchment paper. Repeat with the second dough disk.
Bake for 12-14 minutes, until edges start to turn golden. Remove from oven and allow cookies to cool completely on a wire rack.
Take a cookie base and spread a small spoonful of raspberry chia jam in the center. Top with a matching heart-shaped cookie and gently press down. Repeat until all cookies are assembled and enjoy!