To prepare a strawberry marshmallow blondies recipe, first you need to prepare your ingredients. First, you'll need your flour, baking powder, sugar, and salt. Then, add your dry ingredients to the wet ingredients and mix them together until they're combined. Now, it's time to make the filling. You can use any type of marshmallow, but I like to use marshmallows that have been dipped in chocolate or melted chocolate. I'm not sure what kind of chocolate would be best for this recipe. Using an electric mixer, beat the egg whites on medium speed until stiff peaks form. Add the sugar and vanilla and beat until the mixture is light and fluffy. Pour the batter into the greased and floured muffin tin and bake for 20 to 25 minutes, or until toothpick inserted in the center comes out clean. Allow the muffins to cool before removing them from the pan.