Preheat oven to 350°F. Spread almonds in a single layer on a large rimmed baking sheet. Bake the almonds until toasted and lightly golden, about 8 minutes. Set aside.
Line another large baking sheet with parchment paper, and lightly coat parchment with cooking spray; set aside. Lightly coat a second sheet of parchment with cooking spray; set aside.
Combine butter, sugar, maple syrup, and tap water in a medium saucepan over low. Cook, stirring often, until butter is melted and sugar has dissolved, about 5 minutes, brushing down sides of saucepan with a wet pastry brush as needed. Clip a sugar thermometer to side of saucepan, and increase heat to medium-low. Cook, stirring constantly, until mixture reaches 300°F, 20 to 25 minutes. Stir in vanilla, salt, and baking soda; cook, stirring constantly until combined, about 15 seconds; remove from heat.
Immediately pour sugar mixture onto prepared baking sheet. Place second sheet of parchment over sugar mixture, greased side down. Use a rolling pin to roll sugar mixture into an even layer, 1/8- to 1/4-inch thick. Remove and discard top layer of parchment. Set toffee aside to cool, uncovered, to room temperature, about 30 minutes.
Meanwhile, fill a small saucepan with water to a depth of about 1 inch. Bring to a simmer over medium, then reduce heat to low. Place chocolate in a medium heatproof bowl, and set over saucepan, ensuring bowl doesn’t touch water. Cook, stirring occasionally, until chocolate is melted, about 5 minutes. Remove bowl from saucepan, and set aside to cool slightly, about 10 minutes.
Blot any excess oil off toffee using paper towels. Pour half of the melted chocolate (1/2 cup) over cooled toffee. Spread into an even layer with an offset spatula, and sprinkle with half of the toasted almonds (3/4 cup). Let cool slightly, about 5 minutes, then sprinkle evenly with 1/2 teaspoon of the flaky salt. Set aside until chocolate is firm, 45 to 60 minutes.
Flip toffee and chocolate sheet so that chocolate side is facing down. Blot any excess oil off toffee using paper towels. Pour remaining 1/2 cup melted chocolate over toffee, and spread into an even layer with an offset spatula. (If chocolate has hardened in bowl, place it back over a saucepan of simmering water until re-melted.) Sprinkle chocolate with remaining 3/4 cup toasted almonds. Let cool slightly, about 5 minutes, then sprinkle evenly with remaining 1/2 teaspoon flaky salt. Set aside until chocolate is completely firm, about 2 hours. Break toffee into 2- to 3-inch pieces, and serve.
