For the Sablé Breton: In a large bowl, combine the cold butter cubes, granulated sugar, and sea salt. Using your fingertips or a pastry blender, work the fat and sugar together until the mixture resembles coarse sand. This is the 'sablage' method - coating the flour proteins in fat before liquid is added, which inhibits gluten development and ensures a tender crumb.

Add the all-purpose flour to the sandy mixture. Continue to work the ingredients with your fingertips until the mixture resembles wet sand and no dry flour remains. The fat should have coated all the flour particles.

Gently incorporate the egg yolks into the mixture with a spatula, just until the dough begins to come together. Do not overmix. Shape the dough into a disc, wrap tightly in plastic wrap, and refrigerate for at least 2 hours, or preferably overnight.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

On a lightly floured surface, roll out the chilled dough to about 1/4-inch thickness. Carefully transfer the dough to the prepared baking sheet. You can cut it into desired shapes (rounds, squares) or press it into a tart pan.

Bake for 20-25 minutes, or until the edges are golden brown and the center is lightly colored. Let cool completely on the baking sheet before handling.

For the Candied Citrus: While the sablé bakes, combine the citrus peels, sugar, and water in a small saucepan. Bring to a simmer over medium heat, stirring until the sugar dissolves. Reduce heat to low and simmer gently for 15-20 minutes, or until the peels are tender and translucent and the syrup has thickened slightly.

Drain the candied citrus well and let cool.

For the Italian Meringue: In a small saucepan, combine the 1/2 cup sugar and 1/4 cup water. Heat over medium heat, stirring until the sugar dissolves. Bring to a boil and cook until the syrup reaches 240°F (115°C) on a candy thermometer.

While the syrup heats, whip the egg whites and cream of tartar in a clean, grease free bowl of a stand mixer on medium low speed until foamy.

Once the syrup reaches 240°F (115°C), slowly and carefully pour the hot syrup in a thin, steady stream down the side of the mixing bowl into the whipping egg whites. Avoid hitting the whisk attachment directly.

Increase the mixer speed to high and continue whipping until the meringue is stiff, glossy, and completely cooled to room temperature (about 7-10 minutes).

To assemble: Spread the cooled candied citrus over the baked Sablé Breton base. Generously spoon or pipe the Italian meringue over the citrus, creating decorative peaks. You can optionally torch the meringue for a toasted finish.

Serve immediately or refrigerate until ready to serve. The meringue is best enjoyed fresh.