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Preheat the oven to 350°F with an oven rack in the center position.
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Spread the hazelnuts on a baking sheet and toast, 8 to 10 minutes, tossing halfway through, until they’re golden brown and fragrant. Let the nuts cool on the pan, then transfer to a cutting board and chop finely until the largest pieces are no bigger than a lentil (some of the nuts will turn to dust, which is OK). Transfer the chopped hazelnuts to a small bowl and set aside. Leave the oven on.
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To make the batter: In a small saucepan, combine the butter, sugar, cream, and honey and bring to a boil over medium-high heat, stirring constantly with a heatproof flexible spatula to melt the butter and dissolve the sugar. Once the mixture is at a full, rolling boil, remove the saucepan from the heat and vigorously whisk in the flour and salt until the mixture is smooth and lump-free, then add the vanilla and hazelnuts and stir until combined. Scrape the batter into a medium bowl and set aside.
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Line a large baking sheet with parchment. Scrape about half of the warm batter onto the prepared pan, then place a second sheet of parchment on top. Working over the second piece of parchment, use a small offset spatula to spread the batter into as thin a layer as possible between the two sheets, pressing down firmly and pushing the mixture outward toward the edges (the overall shape isn’t important). Transfer the baking sheet to the refrigerator and chill until it’s firm and set, at least 8 to 10 minutes. (Wrapped airtight, the sheet of batter can stay in the refrigerator for up to 3 days.)
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Remove the baking sheet from the refrigerator and carefully peel away the top piece of parchment, leaving the layer of batter on the sheet pan.
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Bake the florentine batter for 8 to 10 minutes, rotating the pan front to back after 6 minutes, until the batter has transformed into a lacy, weblike slab that’s golden brown across the surface. Remove the baking sheet from the oven and let it cool completely.
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Break the cooled slab into large fragments on the baking sheet, then transfer the pieces to a wire rack.
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Repeat the spreading, chilling, and baking process with the remaining batter using the baking sheet and parchment (the remaining batter will have cooled and firmed up, so you’ll need to press even more firmly with the spatula to spread it). When the second slab of florentines is cool, break it into pieces. Reserve the empty parchment-lined baking sheet.
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To make the chocolate coating: Fill a small saucepan with about 1" of water and bring to a simmer over medium-low heat, then reduce the heat to maintain a gentle simmer. In a medium heatproof bowl, combine the unsweetened chocolate and chocolate hazelnut spread and set it over the saucepan, taking care that the bottom of the bowl doesn’t touch the water (this is called a double boiler). Stir the mixture occasionally with a heatproof flexible spatula until it’s melted and completely smooth, then remove the bowl from the heat and set aside. Let the chocolate mixture cool, stirring occasionally, until it’s thickened.
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Use a small offset spatula to spread a very thin layer of the chocolate mixture across the flat bottoms of the florentines then set the coated florentines chocolate-side down on the reserved sheet pan. (Setting the florentines with the chocolate-side down smooths out the chocolate; after chilling it should easily peel away from the parchment.) When you run out of room on the baking sheet, line another with parchment for the remaining florentines. If your chocolate mixture sets up while you work, reheat it over the double boiler or in the microwave to loosen.
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When all the pieces are coated, transfer the baking sheets to the refrigerator and chill until the chocolate mixture is set, 10 to 15 minutes.
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Storage instructions: The florentines, stored in an airtight container at cool room temperature, will keep for up to 5 days (if stacking them, separate layers between pieces of parchment or wax paper). The florentines will attract moisture from the air and become soft and sticky over time, so make sure the container is truly airtight. If your kitchen is warm, store the florentines in the refrigerator to prevent the chocolate mixture from melting.
