Step 1

Place an oven rack in the middle position. Preheat the oven to 350 degrees F.

Step 2

Grease an 8-inch square baking dish with vegetable oil cooking spray and line with a piece of parchment paper so that it has 2 inches of overhang on 2 sides of the pan.

Step 3

To make the batter: In a large bowl, combine the brown sugar, butter, honey, salt, cinnamon, vanilla extract and lemon zest. Whisk together for 1 minute until smooth. Add the egg and whisk to incorporate. Add the flour and whisk to combine. Fold in the nuts, scrape into the prepared baking dish and bake until puffed and golden brown on the edges but still light in the center, 28 to 30 minutes. Allow to cool in the pan to room temperature.

Step 4

Remove the blondie from the pan and spread the Frosting on top in a swirly layer. Garnish with chopped pistachios and refrigerate for 30 minutes to allow the frosting to get firm for nice slicing. Slice into diamond-shaped bars and serve.

Step 5

To prepare the frosting: In the bowl of a stand mixer fitted with a paddle attachment (or with a hand mixer), cream together the butter and honey until light and creamy, 30 seconds to 1 minute. Add the powdered sugar, lemon zest and juice and salt, then whip for 2 to 3 minutes or until pale and fluffy.