Heat the oven to 375 degrees. Coat a 9-by-9-inch baking pan with baking or cooking spray.
In a large bowl, use an electric mixer on medium to beat the butter and sugar together until light and fluffy. Beat in the egg, salt and vanilla. Add the flour and mix until just incorporated.
Press the dough into the prepared pan. Bake until golden and firm, about 20 to 25 minutes. Let cool in the pan.
Meanwhile, make the caramel. In a medium saucepan over medium, combine the sugar and water, stirring until the sugar is dissolved. Bring to a gentle boil and cook until the sugar reaches a medium brown, occasionally swirling the pan to prevent burning at the edges.
While the sugar cooks, in a small saucepan over medium-low, heat the cream until just warm. Once the sugar is medium brown, add the lemon juice and salt, then gradually whisk in the warm cream. The mixture will bubble vigorously be sure to wear a kitchen mitt to protect your hands.
Scatter the peanuts evenly over the cookie crust, then pour the caramel sauce over them. Let cool for 10 minutes, or until the caramel sets.
Once the caramel has set, make the chocolate sauce. In a medium microwave-safe bowl, heat the cream until boiling. Add the chocolate and let sit for 1 minute. Stir with a wooden spoon until completely smooth and glossy, then pour over the caramel and peanuts.
Refrigerate until firm, about 1 hour. Cut into 16 bars. Store the bars in a single layer in the refrigerator.