Step 1
Combine flour 1/4 cup plus 1 tablespoon butter sugar egg baking powder and vanilla extract in a food processor. Blend ingredients until dough starts sticking together and forms one piece.
Step 2
Wrap crostata dough in plastic wrap and place in the refrigerator for 30 minutes.
Step 3
Preheat the oven to 350 degrees F (175 degrees C).
Step 4
Remove dough from the refrigerator. Roll out dough using a lightly floured rolling pin on a work surface or flatten it directly onto a baking sheet using your fingers to form crostata shell. Pierce shell with a fork to prevent air bubbles during baking.
Step 5
Bake in the preheated oven until sides are golden brown 15 to 20 minutes.
Step 6
Remove crostata from the oven and set aside to cool.
Step 7
While crostata cools pour lemon juice into a heavy saucepan through a sieve and stir in 3 1/2 tablespoons sugar. Heat mixture over medium-low heat until sugar dissolves about 5 minutes. Remove from heat.
Step 8
Beat remaining sugar eggs and flour together in a mixing bowl using an electric mixer or a whisk until white and airy in texture.
Step 9
Pour warm lemon juice mixture into egg mixture slowly whisking constantly. Pour custard mixture back into the saucepan.
Step 9
Heat custard mixture over medium-high heat stirring constantly until thick about 5 minutes. Remove from heat and continue to mix until the uneven texture becomes smooth about 1 minute. Add butter gradually to custard 1 piece at a time stirring constantly until smooth. Let custard cool to room temperature.