Step 1

Place blackberries in a large pot over medium heat. Cook until berries pop and release their juices, about 10 minutes. Using the back of a wooden spoon, press berries through a fine mesh sieve set over a large bowl to collect approximately 1 cup blackberry juice. Discard seeds and pulp. Allow juice to cool to room temperature, about 30 minutes.

Step 2

Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9-inch tart pan with a removable bottom.

Step 3

Combine flour, almonds, and salt in the bowl of a food processor; pulse a few times until everything is combined and almonds are finely ground. Add in melted butter, maple syrup, and vanilla extract. Pulse several more times until mixture is crumbly and resembles wet sand. Pour mixture into the prepared tart pan, pressing it very firmly and evenly into the bottom and sides of the pan.

Step 4

Bake in the preheated oven until edges of the crust are lightly browned, about 20 minutes. Cool on a wire rack while preparing the filling.

Step 5

Whisk together sugar, cornstarch, and salt in a large heat-safe bowl for the filling. Mix in the cooled blackberry juice, eggs, egg yolk, lemon juice, and lemon zest until smooth. Bring a pot of water to a low simmer. Place bowl with blackberry mixture over the pot of simmering water, making sure bowl does not touch water. Cook, stirring constantly, until mixture thickens enough to coat the back of a spoon, about 10 minutes. Remove bowl from heat and stir in butter, 1 piece at a time, ensuring each piece melts before adding the next. Strain custard mixture through a fine-mesh sieve into the prepared crust.

Step 6

Bake in the preheated oven until filling is just set, 20 to 25 minutes. Allow tart to cool completely before removing from pan. Top with additional fresh blackberries just before serving.