For the cupcakes:
Preheat oven to 160°C. Line muffin tins with papers. Combine flour, baking powder and salt; set aside.
Cream butter and brown sugar on medium-high speed until pale fluffy. Add eggs, one at a time, beating until each is incorporated. Add vanilla. Mix and scrape down sides of bowl as needed. Add flour mixture in three batches, alternating with two additions of buttermilk, and beating until combined after each.
Divide batter evenly among lined cups, filling each about halfway full. Bake for about 25 minutes. When done, transfer tins to wire racks to cool 10 minutes; turn out cupcakes onto racks and let cool completely.
For the salted caramel filling:
Melt the sugar over medium high heat in a large pot. Whisk the sugar as it melts and cook until it becomes a deep amber color. Add the butter and stir it in until melted. Pour in the heavy cream (mixture will foam) and whisk until you get a smooth sauce. You may have some lumps but keep stirring until they have melted. Remove from heat and let cool slightly.
Cut a small round piece out of the tops of each cooled cupcake and pour in 1 teaspoon of caramel. Replace the cake piece and set cupcakes aside.
For the salted caramel buttercream frosting:
In a saucepan, stir together granulated sugar and water. Bring to a boil over medium high heat. Cook without stirring until mixture turns a deep amber color. Remove from heat and slowly add in cream and vanilla, stirring until very smooth. Let caramel cool for about 20 minutes, until it is just barely warm and still pourable.
In a mixer fitted with paddle attachment, beat butter and salt together until lightened and fluffy. Reduce speed to low and add powdered sugar. Mix until thoroughly combined.
Scrape down the side of the bowl and add the caramel. Beat on medium high speed until light and airy, and completely mixed (about 2 minutes). Mixture should be ready to use without refrigeration. If your caramel was too hot when added, it will cause your icing to be runny. If this happens refrigerate for 15-20 minutes.
Top caramel-filled cupcakes with frosting.
For the salted caramel rounds:
Lay out a large piece of parchment on your work surface and spray with cooking oil (such as canola). Fill a large tub (or your sink) partially full of ice water.
Put sugar, salt and cold water in a heavy pan; stir over low heat until sugar dissolves. Increase heat and bring syrup to a boil. Lower heat slightly and swirl the pan once or twice as the syrup caramelizes so it will color evenly; do not stir. When the caramel is deep amber, plunge the base of the pan into ice water for about 2 seconds to stop further cooking.
Working quickly, dip a spoon in the caramel and let it fall onto the parchment paper in a drizzle. Move the spoon in a circular motion as the syrup falls from the spoon. Repeat 15 times. When caramel has hardened (it will still be sticky) sprinkle on the large-crystal sugar. Remove from paper and adorn cupcakes.
Recipe courtesy of Heather Baird, author of SprinkleBakes.