Melt the butter in a large pot over medium heat. (Use a pot larger than you think you need, because the caramel will eventually expand quite a bit.) While the butter is melting, crush unsalted peanuts in a bowl with the end of a rolling pin. Once the butter has melted, stir in sugar and water with a heat-resistant spatula. Add a pinch of salt. Bring the mixture to a boil, then add corn syrup. Clip a candy thermometer to the side of the pot and continue cooking until it reads 310 degrees F.
When the caramel is golden and the thermometer registers 310 degrees F, remove the thermometer, add the baking soda, and stir in peanuts. Cook until temperature reads 320 degrees F, then pour the mixture onto a Silpat baking liner and lay another Silpat on top. Use a rolling pin to roll the mixture out to desired thickness and then transfer to a baking sheet. Let cool for 45 minutes.
Melt 2/3 of the chocolate over a saucepan of simmering water. If it begins to feel too thick, add a few drops of canola oil to loosen it up. Check the temperature of the chocolate by testing it on your bottom lip; it should feel warm, but not too hot. Once the chocolate is melted, stir in the remaining 1/3 of chocolate to cool it down. Wipe toffee with a paper towel to remove any excess fat. Pour about half of the melted chocolate over the toffee and spread with an offset spatula. It doesn’t have to be perfectly smooth; you want it to have a bit of a rustic look. Before the chocolate sets, sprinkle with chopped peanuts and transfer to the fridge just until the chocolate has crystallized, 2–3 minutes. Remove from the fridge. Lay another Silpat and baking tray on top of the toffee and flip in one quick motion. Peel off the Silpat and repeat with chocolate and peanuts. Return to the fridge for another 2–3 minutes.
Remove toffee from the fridge and transfer to a cutting board. Break into large or small pieces, pack in a bag, and finish with a bow.