Preheat your oven to 350°F (175°C) and grease your 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium mixing bowl, whisk together the flour, sea salt, and baking soda until well combined. Set this mixture aside.
In a large bowl, combine the melted butter and brown sugar. Stir until the mixture is smooth and well blended. Add the eggs one at a time, mixing well after each addition. Then stir in the vanilla extract.
Gradually add the dry ingredient mixture to the wet ingredients, folding until just combined. Be careful not to overmix, as this could lead to dense blondies.
Gently fold in the mini marshmallows, semi-sweet chocolate chips, and toffee bits.
Pour the batter into the prepared baking pan, smoothing the top with a spatula. Give it a gentle shake to level it out.
Bake in the preheated oven for about 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the blondies from the oven and let them cool in the pan for about 10 minutes. Then, sprinkle a little sea salt on top for an extra flavor boost.
After the blondies have cooled completely, cut them into squares and serve.