Heat oven to 375 F with a rack in the middle position.

Pulse walnuts in a food processor until coarsely chopped. Add flour, maple sugar and salt, and pulse until combined. Add butter, a little at a time, and pulse until the mixture resembles fine breadcrumbs. In a small bowl, beat the egg together with the water, then add it to the food processor and pulse until a dough forms.

Turn the dough out onto a floured surface and knead a few times until the dough holds together when pinched (add more water, 1 teaspoon at a time, as needed). Loosely wrap in plastic or reusable wrap, flatten into a ½-inch-thick disc and refrigerate for 20 minutes.

On a lightly floured surface, roll the dough out so it’s ¼ inch thick. Press it into the tart pan, trimming off any excess. Dock the pastry all over with a fork and freeze for 20 minutes.

Remove the tart shell from the freezer. Cover the surface with parchment paper and fill the shell with pie weights (you can also use dried beans, uncooked rice or granulated sugar). Place the tart pan on a baking sheet and bake until golden and set, 30 to 35 minutes. Remove the pie weights and parchment and continue to bake until light golden brown, 5 to 7 minutes. Transfer to the freezer until cooled, 20 to 30 minutes.

Add raspberries to a small pot, mash with a potato masher or fork, then add the remaining ingredients. Cook over medium heat, stirring often, until thickened, about 10 minutes. Strain through a fine-mesh sieve into a small bowl and let cool.

Spread jam evenly onto the bottom of the tart crust and put the tart in the fridge.

Spread the walnut halves in a single layer on a baking sheet, and bake until toasted and fragrant, 10 to 12 minutes, stirring halfway. When cool enough to handle, finely chop the nuts. Reserve 2 tablespoons for the garnish, and set aside the rest.

Put the butter in a large wide metal bowl and set aside. Attach a candy thermometer to the side of a heavy-bottomed saucepan, then place over high heat and add cream, maple syrup, vinegar and corn syrup (the cream may look curdled, but it will come together as it cooks).

Once the mixture starts to boil, lower the heat to medium-high and continue cooking until the mixture reaches 234 F (112 C), about 30 to 35 minutes. Do not stir or leave the pot unattended.

Once it reaches 234 F (112 C), remove the saucepan from the heat and immediately pour the fudge into the bowl with the butter. Do not scrape any fudge from the sides of the saucepan into the bowl as it may crystallize.

Without touching or stirring the fudge, gently move the bowl in a circular motion so the fudge swirls around and cools. When the fudge has cooled to 130 F (54 C), stir vigorously with a wooden spoon or rubber spatula until the fudge has thickened and lightened in colour, about 5 minutes. Transfer to a stand mixer fitted with the paddle attachment and mix on medium-low until the fudge is the consistency of smooth peanut butter (it may look greasy at first, but the butter will combine with the fudge). Working quickly to avoid seizing, fold in the nuts, then spread the fudge on the jam layer with a small offset spatula. Set aside.

Line a quarter-sheet pan with plastic wrap. Bring 1 cup milk and maple butter to a simmer in a medium saucepan over medium heat, whisking to dissolve the maple butter.

Place ⅓ cup cream in a small bowl, sprinkle with gelatin and set aside to bloom. In a large measuring cup, whisk ½ cup milk, egg yolks, cornstarch and salt until smooth. Whisking continuously, slowly add about a third of the hot milk to the egg mixture. Then pour the tempered egg mixture into the saucepan with the rest of the hot milk and place over medium heat.

Stir with a wooden spoon until the custard thickens, then whisk until it comes to a low boil, 2 minutes. Add the gelatin-cream mixture and whisk until melted. Strain the custard through a fine-mesh sieve onto the prepared sheet pan. Cover with plastic wrap, pressing it gently onto the surface. Refrigerate until fully cooled.

Meanwhile, in a medium bowl, whip the remaining 1 cup cream until stiff peaks form. When the pastry cream has cooled, scrape it into a large bowl and whisk until smooth, then gently fold in the whipped cream. Transfer the mousse to a large piping bag fitted with a #128 rose tip and set aside in the fridge.

Whisk maple sugar, cornstarch, water and corn syrup in a small saucepan over medium-high. Bring the mixture to a boil and cook until thickened, 2 to 3 minutes (monitor it closely so it doesn’t boil over). Remove from heat, transfer to a small bowl and set aside to cool. Brush glaze on raspberries and place them on a wire rack. Set aside.

Remove the tart from the pan and place on a serving plate. Pipe the mousse in a decorative pattern to cover the fudge layer. Arrange raspberries on top of the mousse, sprinkle with reserved walnuts and dust with raspberry powder.