Caramelize the syrup: In a small saucepan (large enough to hold 2 cups of cream), heat ⅓ cup pure maple syrup over medium heat. As it heats, the syrup will begin to bubble; continue heating until the bubbling transforms from fast, small bubbles to slow, large ones, the color deepens, and the aroma smells deeply caramelized, about 5 - 8 minutes.
Add the cream: Immediately stir in 2 cups heavy cream and 1 teaspoon vanilla paste (use caution, the mixture will bubble up vigorously). At this point, the syrup will seize and harden into candy-like shards. Reduce the heat to medium-low, whisking occasionally until the syrup has melted again. Whisk until fully combined; no pieces of maple candy should remain.
Whisk yolks: In a medium bowl, whisk 7 large egg yolks and 1 teaspoon Diamond-Crystal Kosher Salt. Whisking constantly, slowly pour a little bit of the hot cream into the yolks. Once about half the cream has been added, you can whisk in the remaining liquid. Note: Do not add all of the hot liquid at once, as it can scramble the yolks! Strain through a fine-mesh sieve into a large spouted measuring cup or bowl.
Divide among 6 (4-oz) ramekins. Place the ramekins in a large baking dish and pour in hot water halfway up the sides. Bake until the edges are just set and the centers still jiggle slightly, 30 - 40 minutes.
Let the ramekins cool in the water bath until no longer hot to the touch. Transfer to a baking sheet, then refrigerate for 4 - 6 hours or overnight.
Divide 1 ½ tablespoons granulated sugar among the top of the ramekins. Swirl to coat, pouring out any excess sugar, then use a blowtorch to caramelize until amber and crisp. Sprinkle with Flaky salt, then let set for 2 - 3 minutes before serving.
Notes
Inspired by Alison Roman's Maple Caramelized TartBlowtorch: While I have tried brûléeing with a broiler, I don't recommend this option; unless your broiler is very targeted, precise, and strong, it will take so long to caramelize the sugar that the whole custard will heat up. So unfortunately, I do think a blowtorch is necessary here!The size of your ramekins (and how full they are) will dictate the timing of the custard. This is why I go off the jiggle cue. Once the edges are set and the center is still a little jiggly, it's done!