Preheat the oven to 180°C and line the muffin pan with 6 cupcake liners. Set aside.
To unsweetened almond milk, add apple cider vinegar. Mix and set aside.
To a medium-size bowl, add all-purpose flour, cornstarch, baking powder, baking soda, and a pinch of sea salt. Mix with a whisk and set aside.
In a mixing bowl, using a whisk, mix granulated sugar with sunflower oil and strawberry jam. Add vegan buttermilk and mix until well combined.
To the wet ingredients, sift dry ingredients. Mix until just combined. Be careful not to overmix the batter. Next, finely chop fresh strawberries and mix them into the batter.
Divide the batter evenly into 6 cupcake liners. Bake in preheated oven for 20 minutes until a toothpick inserted into the middle comes out clean. If it looks wet, bake for about 5 minutes longer. Transfer to a cooling rack and allow to cool completely before frosting.