Preheat oven to 350 degrees F. Spray a 9×13-inch baking pan with nonstick cooking spray and set aside.
In a medium bowl, whisk together flour, baking soda, and sea salt. Set aside.
In the bowl of a stand mixer, mix the melted butter and brown sugar together until combined and smooth, about 1 minute. Add the eggs and vanilla extract and mix until smooth.
Slowly add the dry ingredients and mix on low, just until combined. Stir in the marshmallows, chocolate chips, and toffee bits.
Press the dough evenly into the prepared pan, smoothing the top with the spatula. Sprinkle blondies with a little sea salt, if desired. I highly recommend it:)
Bake for 30-32 minutes or until the top of the bars are light golden brown and the edges start to pull away from the pan. Cool bars on a wire rack to room temperature. Cut blondies into squares and serve.