Preheat the oven to 350 degrees F.
In a mixing bowl, whisk together all of the dry ingredients until fully combined. Set aside.
In a separate mixing bowl, whisk together the light brown sugar and the melted butter until fully incorporated - about 1-2 minutes. You will know you are done whisking when you cannot distinguish the melted butter from the sugar. There should be no loose butter around the edges of the mixture. Instead, it should be fully whipped into the sugar.
Add the eggs, egg yolk and vanilla, then whisk until incorporated.
Fold 1/4th of the dry ingredients into the wet ingredients and fold with a rubber spatula until combined. Repeat with another 1/4th of the dry ingredients, then repeat with the rest. Mix the batter with a rubber spatula until fully combined.
Butter a square (preferably 9x9) baking dish. Pour 1/3rd of your batter into the baking dish and spread evenly with the back of your spatula. It should be about 1-2 inches thick. Bake for 12-14 minutes, until the edges are fully baked and the middle is still raw and gooey.
Take 1/2 cup of dulce de leche and pour it into the center of the par-cooked blondies. With the back of your spoon spread it evenly to create a dulce de leche layer on top of the previously baked blondie batter.
Dollop the rest of the blondie batter on top of the dulce de leche layer. With the back of your spatula or spoon, smooth it out until it's fully covering the dulce de leche layer. Feel free to have fun with it and create some swirls, but you don't want to fully combine the dulce de leche and the batter.
Finally, dollop the rest of the dulce de leche on top of the final layer of batter and swirl with a spoon. Bake for 35 minutes, until no longer gooey and jiggly. They should slightly rise and look golden brown and crackled on top. Let them cool for 30 minutes-1 hour before digging in for the best structure! If you like it gooey, feel free to just go for it lol.