Preheat oven to 350 ° F. Spray 8 x 8 baking pan with cooking spray, or alternatively, line the pan with parchment paper making sure it comes above the edge of the pan.
In a stand mixer with the paddle attachment, beat butter and sugar on medium speed until light and fluffy (2-3 minutes)
Turn the mixer to low speed and add eggs, one at a time, mixing until thoroughly blended in.
Add lemon juice, lemon zest and Fiori oil (if using) to the mix and blend until just combined.
In a separate bowl, whisk together the flour, baking powder and salt. Fold this dry mix into the lemon batter with a slotted spoon until blended. Do not overmix.
Spread the batter evenly in the pan and bake for 15-20 minutes until just set, or until toothpick inserted in middle comes out clean. Edges will be light brown. Remove from oven and cool on a rack about 15 minutes.
When blondies are completely cooled remove from pan. Whisk together all of the icing ingredients except the zest and spread over the top of the blondies with a knife (can blend the icing ingredients together in mixer if preferred).
With a microplane grater held over the top of the iced blondies, grate lemons and oranges, letting the zest fall lightly on the icing.