Preheat the oven to 350°F and line a the bottom and sides of an 8x8 square baking pan with parchment paper.

Add the sugars and vanilla extract to the bowl of a stand mixer or, if mixing by hand, a large bowl.

In a small saucepan or skillet, melt the butter over medium heat. Keep cooking, stirring occasionally until the butter browns. First it will foam up and then smell toasty with little brown flecks. Pour the browned butter over the sugar and stir a couple of times. Let sit for 10 minutes to cool a bit.

Meanwhile, whisk together the sweet rice flour, millet flour, oat flour, cardamom, baking soda, and salt. Set aside.

Add the egg to the butter/sugar and mix until just combined. With the mixer on low speed, slowly add the dry ingredients to the butter/sugar and beat until just combined. Fold in the white chocolate and mix until evenly distributed.

Dump the dough into the prepared baking pan and use a spatula to spread into an even layer. Bake for 22-24 minutes, until the edges are golden brown and the top is just set, but still soft. Remove from oven and let cool in the pan for a minute or two before carefully using the parchment paper to transfer to a wire rack to cool completely before frosting.

Add the cream cheese and beat until combined. Add about half of the powdered sugar mixing on low speed until combined. Add the vanilla and reduced grapefruit syrup, and beat on medium speed until smooth. (It will seem very wet like it won't incorporate, but it will!). Add the remaining powdered sugar and beat until combined. Fold in the grapefruit zest and salt.

When the blondies are completely cool, spread the frosting over the top and use an offset spatula to create an even surface of frosting. Add sprinkles to your heart's desire. Slice into 16 bars and serve!