Melt the butter over medium heat in a heavy-bottom medium saucepan. Add the water and brown sugar and mix with a spatula until the mixture is starting to gently bubble at the edges. The sugar and butter should be evenly mixed. Place a candy thermometer into the mixture and fix it to the side of the pan.
Over medium heat, bring the mixture just to 285 degrees. Turn the heat off, and quickly add the sifted baking soda to the mixture. Stir the soda into the hot sugar and stir rapidly, but carefully to prevent splashing. Add the vanilla, stir again until incorporated.
Pour the mixture out onto a 12 x 18-inch inch baking pan with 1-inch sides. Using an offset spatula, smooth the mixture out across the surface.
Sprinkle the chocolate onto the candy. Let stand for 5 minutes. Using an offset spatula, smooth the chocolate evenly over the surface.
Then sprinkle the slivered almonds onto the chocolate and gently shake the pan to distribute the almonds. You can gently press the almonds into the chocolate with your fingertips.
Let the toffee cool completely and then cover with plastic wrap. When it is completely solid, slip a flat-edged spatula under the toffee to lift it up and begin cracking the toffee into pieces that can be stored in an air-tight container. I break the toffee into large pieces, and then, when serving, I break any larger pieces into something which can be eaten in a few bites.
This can be stored at room temperature if the container is airtight. You can refrigerate or freeze the toffee but bring it to room temperature prior to serving.