Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until fluffy.

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

In a separate bowl, sift together the flour, baking powder, and salt. Gradually add this dry mixture to the creamed mixture, stirring until just combined.

In a small bowl, mix the filling ingredients: brown sugar, flour, and cinnamon. Set aside.

Roll out half of the dough on a floured surface until it's about 1/8 inch thick. Cut into rectangles (about 3x4 inches).

Place half of the rectangles on the prepared baking sheet. Spoon a small amount of the filling mixture in the center of each rectangle, leaving space around the edges.

Roll out the remaining dough and cut into rectangles, then place these over the filled rectangles. Press the edges to seal.

Brush the tops with the beaten egg and sprinkle with decorative sprinkles if desired.

Bake for 12-15 minutes or until golden brown. Allow them to cool before serving.