Make the brown butter: Heat a medium light colored (stainless steel) saucepan over medium heat and add the butter. Bring it to a boil and let it roll for about 2 minutes. The butter will bubble up as water evaporates from it. Start stirring the butter or swirling the pan to closely watch it change color. You don’t want to burn it! Take the butter off the heat as soon as its milk solids appear golden and the kitchen smells nutty and heavenly. Off the heat, keep on stirring for a few seconds so you can make sure it’s the perfect color, then pour it into a large mixing bowl and set aside to cool for 10 minutes.
In a small bowl, whisk the flour and baking powder together. Set aside.
Add the brown sugar to the mixing bowl containing the brown butter. Using an electric mixer, beat the sugar and brown butter together for 2 minutes to thoroughly incorporate. Add the egg and beat until light in color, about 3 minutes. Add the Calvados or Cognac and vanilla extract and beat to incorporate. Add the reserved flour mixture and beat just to incorporate. Using a spatula, fold in the diced apple and chopped walnuts. Transfer the mixture to the prepared pan. The mixture will be quite thick so you’ll need to use a spatula to spread it to the edges of the pan.
Bake for 25 minutes, or until the top is set and the edges are barely golden. Set the pan on a wire rack and let cool completely.
For the salted butterscotch glaze: In a medium saucepan, combine the butter, brown sugar, cream, and salt in a medium saucepan over medium heat, stirring from time to time. When the butter is melted, raise the heat to medium-high and bring to a boil. Whisking constantly, boil for 2 minutes. Remove from the heat and keep on whisking for 1 minute. Transfer to a large mixing bowl and let cool to room temperature, about 20 minutes.
Add 1/4 cup (60 ml) of the powdered sugar, whisking until incorporated. Gradually add more powdered sugar, whisking constantly to incorporate. The glaze is ready to use as is: at room temperature, the glaze will be pourable and beautifully drip to the sides of the brown butter apple blondies. If you're looking for a thicker glaze you can spread over the blondies like you would an icing, refrigerate the glaze for about 20 minutes first. Lightly sprinkle the glazed brown butter apple blondies with sea salt flakes. Cut into squares, or refrigerate for 30 minutes to fully set the glaze and make neater squares.
Always serve the brown butter apple blondies at room temperature. If they're stored in the refrigerator, take them back out to room temperature about 20 minutes before serving.
Store the brown butter apple blondies in an airtight container in the fridge for up to 3 days.
Recipe Credit: Marie Asselin, FoodNouveau.com
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