Preheat oven to 325F and line a 2-quart baking dish with parchment paper.
Combine butter, milk, and coconut sugar in a small saucepan over low heat, stirring until butter is melted. Remove from head and set aside.
Combine oat flour, almond flour, baking powder and salt in a large mixing bowl. Stir to combine.
In a separate mixing bowl, whisk eggs (skip steps 4 and 5 if using an egg replacement and add to step 3).
Very slowly whisk in small amounts of milk and butter mixture into the eggs, whisking the entire time. Continue until all of the butter and milk mixture is whisked with eggs (see note).
Pour wet ingredients into the bowl of dry ingredients and whisk until smooth.
In a separate bowl, stir together filling ingredients until well-combined.
Using a spatula or spoon, spread half of the blondie batter into the prepared pan, smoothing down into a flat layer.
Next, spread filling on top of first layer in the pan.
Lastly, spread the remaining blondie batter on top of filling, smoothing down with the spatula.
Lightly brush with more melted butter (optional).
Bake in the oven for 22-26 minutes. Remove from oven and let cool.
Whisk together icing ingredients until smooth and pour on top of cooled blondies. Slice into 12 squares and serve immediately.