Preheat oven to 335°F.
Prepare an 8x8-inch metal baking pan: butter the sides and bottom, then line the bottom with a square of parchment or wax paper.
Melt the butter over low heat in a small saucepan and set aside to cool slightly.
In a small bowl, beat the whole egg and egg yolk with a fork until frothy.
In a medium mixing bowl, combine the melted butter, beaten eggs, coconut sugar, Lakanto, and vanilla. Whisk until smooth.
In another medium bowl, whisk together the oat flour, coconut flour, cornstarch, baking powder, and salt.
Pour the dry ingredients into the wet ingredients. Whisk until just combined — do not over-mix.
Fold in the white chocolate chips with a spatula.
Let the batter sit on the counter for 2–3 minutes so the oat and coconut flours can absorb the liquid.
Pour the batter into the prepared pan and smooth the top.
Bake for 20–25 minutes, until the center is just cooked through when tested with a fork. For extra chewy blondies, pull them at the lower end of the range.
Let cool in the pan for at least 15 minutes before slicing into squares.