Preheat your oven to 350°F (175°C) and line the baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large bowl, whisk together the melted butter and brown sugar until the mixture is smooth and no lumps remain.
Add the eggs and vanilla extract to the butter mixture, whisking until the batter is glossy and well-combined.
Fold in the flour, baking powder, and salt using a spatula until just incorporated; do not overmix, as this will lead to a tough texture.
Gently stir in the chocolate chips.
Spread the batter evenly into the prepared pan and bake for 25 to 30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Remove from the oven and let the bars cool completely in the pan on a wire rack.
Once cool, lift the blondies out using the parchment paper and cut them into 24 even squares.
Spread a generous tablespoon of your chosen chocolate filling onto the flat bottom of one square, then top with a second square to form your sandwich.