Preheat the oven to 350℉ and grease a 9 × 13-inch cake pan with vegan butter or neutral flavored oil and line it with parchment paper.
In a bowl, toss the raspberries with 1 tbsp of flour. Set aside.
In a large bowl or stand mixer with the paddle attachment, rub the sugar and lemon zest together. Then add in the melted plant-based butter, plant-based yogurt, vanilla bean paste or vanilla extract, and sea salt. Whisk until combined. Then add in the cornstarch, baking powder, and baking soda, and whisk again. Finally, add in the flour and plant-based free milk, and whisk just until combined.
Add the raspberries and gently fold until they are evenly distributed throughout the batter. This is a thick batter, and raspberries are very delicate, so some of the raspberries will break as you mix them in. This is normal! Just try to be as gentle as possible so that not all of the raspberries break while being mixed in.
Pour the cake batter into your prepared pan. This is a thick batter, so take a spatula and spread the cake batter into an even layer in your pan. Bake your cake for 45 to 50 minutes or until the toothpick comes out relatively clean, if you get some raspberry juice or pieces on the toothpick, the cake can still be done, you just hit a raspberry with the toothpick when testing. Try another area of the cake. Let the cake cool in the pan for 10 minutes, then transfer it to a drying rack to cool completely before frosting.