Preheat your oven to 350°F (175°C) and prepare a cupcake tin by lining it with paper liners.

In a large mixing bowl, combine the softened unsalted butter and granulated sugar and beat until light and fluffy. Add eggs and vanilla extract, mixing until smooth.

In a separate bowl, whisk together flour, baking powder, baking soda, salt, and nutmeg.

Gradually combine dry ingredients with the wet mixture, alternating with buttermilk until just combined.

Mix brown sugar, cinnamon, and optional chopped nuts in a small bowl for the filling.

Layer one tablespoon of batter into each paper liner, followed by one tablespoon of cinnamon filling, and top with another tablespoon of batter.

Bake for 18-20 minutes, or until a toothpick comes out clean. Cool for 5 minutes before transferring to a wire rack.

Beat softened cream cheese and unsalted butter until smooth. Gradually mix in powdered sugar, then add vanilla extract.

Adjust frosting consistency with milk as needed.

Once cupcakes are completely cool, frost using a piping bag or spatula.