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Preheat the oven to 325°F. Lightly grease an 8” square pan, line the bottom with parchment, and lightly grease the parchment.
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To brown the butter: In a saucepan set over medium heat, melt the butter. When fully melted the butter will start to sizzle and may spatter and pop a bit.
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Continue to cook the butter, swirling the pan regularly until it’s a rich golden brown, with brown bits at the bottom of the pan. The sizzling will have stopped, and there may be a layer of foam on the surface.
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To make the blondies: Remove the pan of butter from the heat and pour the liquid butter (and brown bits) into a heatproof medium-sized mixing bowl.
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Add the baking sugar alternative to the browned butter and whisk to combine.
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Add the egg, egg yolk, and vanilla extract.
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In a small bowl whisk together the flours, baking powder, and salt.
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Stir the dry ingredients into the butter mixture, followed by the sugar-free chips and sprinkles.
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Spread the mixture evenly in the pan and top with additional sprinkles and baking chips, if using.
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To bake the blondies: Bake the blondies for 20 to 24 minutes, until they're set in the middle, golden brown, and just starting to pull away from the sides of the pan.
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Remove them from the oven, loosen their edges with a table knife, and cool completely before cutting.
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Storage information: Store any leftovers, well wrapped, at room temperature for up to a week; freeze for longer storage.