Preheat oven to 350°F Spread hazelnuts on a pie plate. Bake until golden brown (around 12 minutes). Transfer hot hazelnuts to a mini-food processor and pulse until mixture forms a paste. Add chocolate and pulse until melted.
Cook half-and-half, brown sugar, and salt in small saucepan over medium heat, stirring, just until mixture comes to a boil and brown sugar dissolves, about 3 minutes. Pour into glass measure. With food processor running, add half-and-half mixture to chocolate-nut mixture through feed tube and puree. Transfer to bowl and let cool until spreadable.
Spray one side of bread slices with nonstick spray. Put 4 slices, sprayed side down, on sheet of wax paper and spread each with 2 Tbsp chocolate nut mixture. Top with remaining slices, sprayed side up.
Heat panini maker or waffle maker according to manufacturer's directions. Add panini, in batches if necessary, and cook until golden brown, around 2 minutes. Cut each panino into thirds.