First, make the blondies. Preheat the oven to 350°F. Line an 8 x 8-inch square baking pan with parchment paper on all sides. Set aside.
In a large mixing bowl, whisk the melted butter, brown sugar and honey together. Add in the egg, egg yolk and vanilla extract and whisk to combine.
Add the flour, baking powder, and salt. Use a silicone spatula to fold until combined and no streaks of flour remain. Gently fold in the chopped nuts, mixing only until combined.
Transfer the batter into the prepared pan. spreading to the edges and evening out the top. Place in the preheated oven and bake until the blondies are golden brown and a knife inserted into the center comes out mostly clean, about 25 minutes.
While the blondies are baking, make the honey caramel sauce. In a small saucepan over medium heat combine the butter and honey. Use a vegetable peeler to peel off a 1 inch strip of lemon rind and add to the saucepan. Set the lemon aside for later–you’ll use some of it’s juice! Simmer the butter and honey mixture over low-medium heat until the mixture is a dark brown and has reduced in half, about 15 minutes. Add the heavy cream, 2 teaspoons of lemon juice, cinnamon and salt and cook until the mixture darkens in color and it thickly coats the back of a spoon, about 10 more minutes. Remove from the heat and allow to cool slightly. The blondies should also be done cooking now–take them out of the oven and place on a wire rack to cool until the pan is cool to the touch.
Sprinkle the chopped pistachios and walnuts over the top of the blondies and pour the caramel sauce over the nuts, using a rubber spatula to spread into an even layer. Cover the pan with plastic wrap and place in the fridge to set completely, at least 2 hours!
Once the caramel is set, use a very sharp knife to cut the blondies into squares (or cut the squares in half to make triangles). Serve and enjoy!