Preheat oven to 350 degrees Fahrenheit. Spray an 8x8 baking pan with cooking spray and line with parchment paper.
In a medium bowl, add cornstarch and water. Whisk to combine. Add melted butter, agave, cane sugar, vanilla extract, and almond extract. Whisk again to incorporate.
Add almond flour, coconut flour, salt, and baking soda. Using an electric mixer, mix on medium speed until dough starts to form, about a minute. If the dough is not sticking together, you may need to use your hands to press together and incorporate into one big ball of dough.
Transfer dough to prepared pan.
Using an extra piece of parchment paper, flatten dough into an even layer, making sure all corners are filled.
Bake for 14-15 minutes. Remove from oven and allow to cool on wire rack.
While bars are cooling, rinse out and dry bowl, and add frosting ingredients. Using electric mixer again, beat until frosting becomes smooth.
When ready to frost, gently lift bars out of pan using parchment flaps and place on a cutting board. Using a spatula, frost bars and add sprinkles if desired. Cut into large bars or bite-sized squares.
Store in an airtight container.