Heat oven to 350°F. Toast pecans and almonds on a sheet pan until fragrant, 6–8 minutes.
Line a 9x13-inch pan with parchment. Lightly butter the parchment for easy release.
Combine butter, sugar, water, corn syrup, and kosher salt in a heavy saucepan.
Cook over medium heat, stirring until sugar dissolves. Boil without stirring to 300°F, deep amber.
Remove from heat and carefully stir in vanilla. Pour onto prepared pan and spread evenly.
Scatter chocolate over hot toffee. Rest 2 minutes, then spread smooth.
Sprinkle toasted nuts and flaky salt on top. Press gently so they adhere.
Cool until set. Break into pieces and store in an airtight container.