Step 1- Make the Crust
Position a rack in the center of the oven and heat to 350°F. Process the graham crackers in a food processor until fine crumbs form. Add the salt, sugar, and melted vegan butter, and process again until well combined.
Step 2- Shape and Bake the Crust
Press the crumbs into the tart pan, covering the bottom and pressing some up around the sides to form a rim about 1/4-inch thick. Bake until golden brown and fragrant, about 10 minutes. Cool completely on a rack.
Step 3- Infuse the Milk
In a medium saucepan, combine the plant milk, 1/3 cup sugar, salt, and scraped vanilla seeds and pod. Bring to a simmer over medium-high heat, but don't let it boil. Turn off the heat and let sit for 15 minutes to infuse. If using vanilla extract instead, skip this infusing step and add it later.
Step 4- Make the Cornstarch Slurry
In a medium bowl, whisk together the cornstarch and remaining 1/4 cup sugar until fully combined.
Step 5- Combine and Cook
Remove the vanilla pod from the milk and bring it back to a simmer. Slowly whisk about half the hot milk into the cornstarch mixture to temper it, then pour everything back into the saucepan. Cook over low heat, stirring constantly with a wooden spoon, until thickened, about 5 to 7 minutes. Switch to a whisk if needed and beat vigorously until smooth and thick like pudding.
Step 6- Finish the Custard
Remove from heat and whisk in the vegan butter and vanilla extract, if using. Strain through a fine mesh sieve if there are any lumps. Press plastic wrap directly onto the surface and refrigerate until cold and set, at least 3 hours or up to 2 days.
Step 7- Assemble the Tart
Whisk the cold custard until light and smooth, about 1 to 2 minutes. Pour into the cooled tart shell and top with mixed berries and mint sprigs as desired.
Step 8- Glaze and Serve
For a shiny finish, brush the berries with a little jam thinned with hot water. Refrigerate for up to a day before serving, adding the fruit just before serving for the freshest look.
