Crust and Crumb Topping:

• 1/2 c. unsalted butter, melted, plus more for the pan
• 2 pkg. Effie's Hazelnut Biscuits, crushed
• 1/4 c. sugar
• 1/4 t. ground cinnamon
• 1/4 t. kosher salt

Filling:

• 1 lb. ripe but firm black plums
• 1 T. all-purpose flour
• 1 T. sugar
• Zest of 1 medium orange
• 1 large egg, lightly beaten
• 1 large egg yolk
• 1/3 c. heavy cream
• 1/4 c. milk
• 1/4 t. kosher salt
• 1/8 t. ground cardamom

Preheat the oven to 350F. Butter a 9-inch springform pan. Set aside.

Place the biscuits into a food processor and pulse until you get fine crumbs.

Transfer the crumbs to a medium bowl and stir in the sugar, cinnamon, and salt until combined. Pour in the melted butter and stir until the crumbs are evenly moistened and clump together when pressed.

Press about 2/3 of the crumb mixture into the bottom of the prepared pan and about 1 1/2 inches up the sides to form the crust. Press the crumbs neatly down and up the side, forming a clean inner corner. Set the remaining crumb mixture aside for the topping.

Bake the crust until it appears set, 12-15 minutes. Remove from the oven and set aside to cool. 

Slice the plums in half and remove the pits. Slice larger plums into eighths and smaller ones into quarters. Arrange on top of the cooled crust. 

In a medium bowl, rub the orange zest into the sugar with your fingers until fragrant. Whisk in the flour, then the egg, egg yolk, heavy cream, milk, cardamom, and salt. 

Pour the custard over the fruit, then sprinkle with reserved crumb mixture. 

Bake the tart until the custard has set and is slightly golden, 45-50 minutes. 

Let rest at least 25 minutes before cutting, if serving warm. The tart can be served warm or at room temperature!