Spread the almonds on a rimmed baking sheet and cook, stirring once, until toasted, about 10 minutes and set aside.
Lay a silicone baking mat on a rimmed baking sheet (or line the pan with aluminum foil and lightly butter). Spread the butter on the underside of a large offset metal spatula. Whisk the cocoa and baking soda together in a small bowl.
Put the sugar, corn syrup and 3/4 cup water in a deep medium saucepan and attach a candy thermometer to the side. Cook over medium heat, stirring gently with a wooden spoon, just until the sugar dissolves. Increase the heat to high and boil the mixture until it reaches 310 degrees F or hard-crack stage.
Remove the saucepan from the heat and carefully whisk in the cocoa mixture until combined. Stir in the toasted nuts with a wooden spoon. Working quickly, pour the hot brittle onto the prepared baking sheet and spread it as thin as possible with the buttered spatula. Cool for 5 minutes, then loosen the brittle from the pan with the spatula. Cool completely and then break into pieces. Store at room temperature in an airtight container for up to 5 days.
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