Line a rimmed baking sheet with parchment paper. Measure out vanilla and almonds; set aside.
In a large Dutch oven, bring butter, sugar, water, and salt to a boil, stirring constantly. Once butter and sugar are melted, insert your candy thermometer, making sure it is covered by the mixture but not touching the bottom of the pot. Cook, continuing to stir, until mixture reaches 298 degrees (hard crack stage). Remove from heat and quickly stir in vanilla, then pour mixture onto the prepared baking sheet. Smooth out with a spatula, sprinkle on almonds, then set aside to cool for at least an hour.
Once toffee has cooled, place chocolate in a microwave-safe bowl. Microwave on high in 30-second increments, stirring after each. Heat until just melted, then pour over cooled toffee and spread even with a spatula. Allow to harden, then break into pieces and serve.