Preheat oven to 350°F. Line a 12-cup muffin pan with liners and set aside.
Whisk to combine flour, cocoa powder, and baking soda in a medium bowl.
Place butter, granulated sugar, brown sugar and salt in the bowl of a stand mixer fitted with a paddle attachment. Cream on medium speed until light and fluffy, 3-4 minutes. Add the eggs, one at a time, mixing well after each addition. Scrape down the sides of the bowl as needed. Mix in the vanilla extract.
Reduce mixer speed to low. Alternate adding dry ingredients and buttermilk in three batches, starting and ending with dry ingredients. Mix until just combined.
Fill cupcake liners ⅔ full of batter. Bake for 20-22 minutes, or until a toothpick inserted in the center comes out clean. Let cupcakes cool 5 minutes in the pan, then transfer to a wire rack to cool completely.
Heat soft caramel candies and heavy cream in a small saucepan over low heat. Cook, stirring often, until melted and smooth. Set aside to cool completely.
Place pretzels in a small bag and use a rolling pin to crush them into small pieces.
Combine butter and powdered sugar in the bowl of a stand mixer fitted with a paddle attachment. Cream until smooth. Beat in heavy cream. If needed, add more powdered sugar to thicken or a teaspoon of heavy cream to thin.
Frost cooled cupcakes with buttercream frosting in a simple swirl using a large, plain piping tip. Use an offset spatula to smooth the top of the frosting and create swirls on the surface. Spoon cooled caramel into center and sprinkle with crushed pretzels. Repeat with remaining cupcakes.