• Preheat your oven to 400°F (200°C).
• In a heatproof bowl, stir together the cubed butter, water, canola oil, sugar, and salt.
• Bake until the butter is bubbling and lightly browned at the edges, about 20 minutes.
• Remove from the oven and stir in the flour until the dough comes together.
• Press the dough into the bottom and up the sides of five 4-inch tart pans (or one large 9-inch tart pan).
• Use a fork to prick the dough all over.
• Bake until the crust is cooked through, 10-12 minutes.
• Let cool.

Make the Filling:

• In a 4-quart saucepan, whisk together the sugar, cornstarch, flour, salt, and egg yolks until smooth.
• Stir in the milk and place the saucepan over medium heat.
• Cook, stirring constantly, until the mixture thickens, about 15 minutes.
• Remove from heat and slowly whisk in the chilled butter until smooth.
• Stir in the vanilla extract.
• Transfer the pastry cream to a bowl and cover with plastic wrap, pressing it directly on the surface to prevent a skin from forming.
• Chill until set, at least 2 hours.

Assemble the Tartlets:

• Once the pastry cream is set, spread it evenly over the cooled tart shells.
• Garnish with fresh blackberries.
• Chill the tartlets until ready to serv