In large bowl, mix flours, protein powder, sweetener and salt and mix well.
Combine cashew butter and syrup in microwave bowl and heat until melted about 30 seconds. Stir together until smooth.
Pour mixture into dry and mix until well-combined, about 3 minutes. Add vanilla and milk and mix well until a firm batter is formed. If necessary, add more milk by the tablespoon until the right density is reached.
Transfer dough to lined baking dish and press down firmly to create an even layer.
Refrigerate for at least 30 minutes to firm. Cut into square or rectangle pieces and devour!