Preheat your oven to 350°F (175°C) and prepare a 9x13 inch baking pan by greasing and flouring it, or lining it with parchment paper.

Gather your mixing bowls, whisk, and rubber spatula.

Whisk together the all-purpose flour, baking powder, baking soda, salt, and optional spices like cinnamon and nutmeg in a medium bowl.

In a large mixing bowl, cream together softened unsalted butter, granulated sugar, and brown sugar for about 3-5 minutes until light and fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture in three parts, alternating with the buttermilk in two parts.

Pour the batter into the prepared baking pan and bake for 30-35 minutes.

Remove the cake from the oven and let it cool in the pan for about 15 minutes.

Transfer it to a wire rack to cool completely.

Melt the caramel candies with milk or heavy cream in a small saucepan over low heat.

Beat the softened butter until creamy, then gradually add the sifted powdered sugar, followed by the cooled caramel mixture.

Once the cake is cool, frost the top generously with the prepared buttercream and sprinkle flaky sea salt on top.