Preheat the oven to 350°F. Line 2 cookie sheets with parchment paper.
Whisk the flour, cinnamon, baking powder and salt together.
In a separate bowl, beat brown sugar and butter together until creamy. Add the egg and vanilla and mix until combined. Mix in the dry ingredients until well combined.
Shape the dough into 24 balls. There should be cookie dough left over. Place the balls 2 inches apart on the prepared cookie sheets. Use a spoon or your thumb to make a large 1/2 inch imprint in each ball.
Place the dulce de leche into a piping bag or ziplock bag. Cut off the edge of the bag and pipe dulce de leche into each thumbprint hole until 3/4 full. Cover dulce de leche with a piece of the reserved dough. Gently roll the cookie balls in the cinnamon sugar mix until coated, then place back on the cookie sheet.
Bake in the preheated oven for 10-15 minutes, until the edges are lightly browned. Let cool completely before eating.