In a medium saucepan, combine your chosen fresh or frozen raspberries with granulated sugar and a splash of water. Cook over medium heat while gently mashing the berries with a fork until the mixture thickens, about 5-10 minutes. Stir in cornstarch, ensuring it's fully dissolved.

In a mixing bowl or food processor, combine all-purpose flour, vegan block butter, and powdered sugar. Blend until the mixture resembles coarse crumbs, then gradually add cold water, stirring until the dough forms a cohesive mass. If the dough appears too dry, add a little more water one tablespoon at a time.

On a lightly floured surface, roll out the dough to a thickness of approximately 5 mm. Use a round cookie cutter or the rim of a glass to cut out circles that will fit your mini tart tins. Carefully place each circle into the greased tins.

Once your pastry shells are ready, spoon about one tablespoon of the cooled raspberry jam into each shell, leaving a bit of space at the top.

Preheat your oven to 180°C (350°F) and bake the filled tart tins for about 20 minutes, or until the edges of the pastry turn golden brown.

After baking, remove the Vegan Jam Tarts from the oven and allow them to cool completely in the pans.