Chop the semi-sweet chocolate into small pieces and melt three-quarters of it in a double boiler over low heat, stirring gently until smooth and glossy. Keep the remaining chocolate for topping.
Pour a spoonful of the melted chocolate into each paper-lined cavity of your mini muffin tin. Use the back of a spoon or a small spatula to spread the chocolate along the sides, creating an even shell. Chill in the fridge for about 10 minutes until set.
Meanwhile, make the caramel by melting butter in a small saucepan over medium heat. Add the brown sugar and a pinch of sea salt, stirring constantly until the mixture thickens slightly and becomes fragrant, about 3-4 minutes. Remove from heat and stir in the heavy cream until smooth.
Take the chocolate shells out of the fridge and spoon a small amount of caramel into each, being careful not to overfill. Return to the fridge for another 10 minutes to let the caramel set.
Mix the creamy peanut butter with a pinch of sea salt until smooth. Using a small spoon or piping bag, carefully add a layer of peanut butter over the caramel in each cup, pressing it down gently. Chill again for 10 minutes to firm up the peanut butter layer.
Re-melt the remaining chocolate if it has cooled and thickened. Pour over each peanut butter layer, covering it completely. Use a spatula to smooth the tops and sprinkle with a tiny pinch of sea salt or crushed pretzels for extra texture. Chill the assembled cups for at least 30 minutes until fully hardened and glossy.
Once set, remove the cups from the fridge. Gently peel away the paper liners or turn out the cups if using silicone molds. Let sit at room temperature for about 5 minutes before serving to enhance shine and flavor.