Preheat the oven to 350°F (175°C) and line an 8x8-inch (20cm) baking pan with parchment paper.
In a medium bowl, sift the sorghum flour, tapioca starch, quinoa flour, psyllium husk powder, baking powder, cinnamon, and salt.
In another bowl, whisk together the melted butter, brown sugar, eggs, and vanilla extract until smooth.
Core the apples and dice them without peeling.
Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in the diced apples.
Spread the batter evenly in the prepared baking pan.
Bake for 40 to 45 minutes, or until a toothpick inserted into the centre comes out clean.
While the bars are baking, make the caramel maple icing
In a medium-sized saucepan, whisk together the granulated sugar, butter, half and half cream, icing sugar, salt, and maple syrup until smooth and combined.
Over medium heat. Continue whisking as the mixture heats, ensuring the butter melts and the sugar dissolves completely.
Allow the icing to come to a gentle boil while whisking for 1-2 minutes, until it thickens slightly.
Remove the saucepan from the heat and let the icing cool for a minute or two. If i the consistency is too thick, just add a little more cream to make it more spreadable.
Spread the warm icing over the cooled Caramel Apple Bars. Allow the icing to set before slicing.
Allow the bars to cool completely before cutting into squares.