In a mixing bowl, cream together the softened butter and brown sugar until light and fluffy.

Add the egg and vanilla extract, mixing until well combined.

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

Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms.

In a small bowl, combine brown sugar, cinnamon, and melted butter, mixing until well combined.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Roll out the cookie dough on a floured surface to about 1/4 inch thick.

Use a cookie cutter to cut out shapes and place half on the prepared baking sheet.

Spoon a small amount of filling onto each cookie shape, then top with another cookie shape, sealing the edges.

Bake in the preheated oven for 12-15 minutes or until the edges are golden brown.

Remove from the oven and let cool on a wire rack.

In a bowl, whisk together powdered sugar, milk, and vanilla extract until smooth.

Drizzle the icing over the cooled cookies.