Sous vide sweet potato casserole with pecan streusel and whiskey marshmallow

Instead of boiling, sous vide the sweet potato cubes: Harold McGee has pointed out that 170F activates an enzyme that makes them naturally sweeter, so less sugar is needed.

Subdivide the potatoes into two smaller bags, optionally with a cinnamon stick or two, and cook in a 170F water bath for one and a half hours, weighing them down to keep them submerged.

Make brown butter: melt five ounces of unsalted butter in a medium saucepan, swirling constantly over medium heat until the milk solids separate and turn nutty brown, then cool.

Put the cooked potatoes in a large bowl with the brown butter and all its brown bits, four and a half ounces of light brown sugar, four ounces of creme fraiche, two tablespoons of maple syrup, a tablespoon of vanilla paste or a teaspoon of extract, half a teaspoon ground cinnamon, a quarter teaspoon freshly grated nutmeg, a generous pinch of kosher salt and three beaten egg yolks; mix with a paddle until evenly distributed.

Puree in a food processor in batches, about 60 seconds each, until dreamily smooth - the only way to break down the fibre.

Spread evenly in a nine by 13 casserole generously greased with unsalted butter. Do not taste it; it contains raw egg.

Make pecan streusel: combine four and a half ounces of all purpose flour, six ounces of brown sugar and four and a half ounces of roughly processed pecans.

Add two and a half ounces of melted butter, and more as needed, until the streusel has the texture of very wet sand; press together and crumble over the sweet potatoes.

Bake at 325F for 25 to 30 minutes.

Dissolve half an ounce, or two packets, of unflavored gelatin in five ounces of cold water and set aside to bloom.

In a small saucepan combine six ounces of sugar, two ounces of water, one ounce of light corn syrup and two ounces of scotch whiskey, whisk gently, cover and bring to a simmer over medium heat.

Uncover and cook to 255F, the firm ball stage, then kill the heat, add a splash of whiskey and cool to 212F.

Melt the gelatin over a double boiler and combine it with the 212F sugar syrup in the bowl of a stand mixer.

Whip with the wire whisk attachment on medium high, starting slow, for 10 to 15 minutes until nearly quadrupled in volume and at stiff peak stage.

Pour into a casserole and cut into cubes for traditional marshmallows, or pipe onto the slightly cooled sweet potato casserole before it sets - cool enough that the marshmallow does not melt.

Let it set for an hour and brown in a 350F oven, or torch it if the casserole is still hot.