Heat oven to 350°F. Slice each croissant in half lengthwise. Line a large rimmed baking sheet with parchment and lay the croissant halves cut side down in a single snug layer. Top with a second sheet of parchment, then nest a second baking sheet on top to press them flat.
5 medium croissants
Bake for 13-15 minutes, until the croissants are deep golden and crisp. Lift up the top pan to check if needed. Leave them on the sheet, but remove the top of off.
Meanwhile, make the toffee. In a saucepan over medium heat, melt the butter and brown sugar together, stirring until smooth. Bring to a boil and cook, stirring constantly, for 3 to 4 minutes, until glossy and slightly thickened. Off the heat, stir in the fine sea salt and vanilla.
1 cup unsalted butter, 1 cup light brown sugar, ¼ teaspoon fine sea salt, 1 teaspoon vanilla extract
Working quickly, pour the hot toffee evenly over the pressed croissants and spread to the edges. Gently press the croissants back down into the toffee so they anchor in (using your wooden spoon or spatula).
Scatter the chopped dark chocolate over the hot toffee. Let sit 3 to 5 minutes until melted and glossy, then spread smooth with an offset spatula.
7 to 8 oz dark chocolate
Sprinkle generously with flaky sea salt (I also decorated mine with gold sheet pieces). Chill at least 30 minutes until fully set, then break into shards. Store cold.
Flaky sea salt