In a medium saucepan over low, heat the butter, sugar, and a splash of water (about 1 tablespoon) until melted and combined, stirring often.
Place a candy thermometer in the pan, and continue to cook, stirring occasionally, until the mixture reaches 290 F.
Turn off the heat, and stir in the vanilla, salt, and baking soda.
Immediately pour the mixture onto the prepared baking sheet, spreading it evenly to a little over ¼-inch thickness. Set the toffee aside to cool completely.
In the microwave, heat ½ of the chocolate ⅔ of the way.
Stir to melt it the rest of the way.
Spread the melted chocolate evenly over the cooled toffee and chill until firm.
Flip and repeat with the remaining chocolate on the remaining side, sprinkling with almonds and sea salt, if using, before the chocolate sets.