Preheat the oven to 350°F (175°C). Lightly spray a 9x13-inch baking pan with nonstick cooking spray and set aside.
In a medium bowl, whisk together the all-purpose flour, baking soda, and 1 teaspoon sea salt until evenly combined; set aside.
In the bowl of a stand mixer (or a large bowl with a hand mixer), beat the melted and slightly cooled butter with the packed light brown sugar until smooth, about 1 minute.
Add the eggs, one at a time, then beat in the vanilla extract until the mixture is smooth and uniform.
With the mixer on low, slowly add the dry flour mixture and mix just until combined; avoid overmixing.
Fold in the mini marshmallows, semi-sweet chocolate chips, and toffee bits until evenly distributed through the batter.
Spread and press the batter evenly into the prepared 9x13 pan, smoothing the top with a spatula. Sprinkle a little sea salt over the top if desired.
Bake for 30–32 minutes, until the top is light golden brown and the edges begin to pull away from the pan.
Cool the blondies in the pan on a wire rack to room temperature, then cut into squares and serve.