Combine the water, yeast and sugar in a bowl. Rest for 5 minutes.
In a medium-sized bowl, gently whisk together the flour, cornstarch, baking powder, salt, and baking soda. Set this dry mix aside for later.
In a large mixing bowl (using an electric mixer is easiest!), beat the softened butter with both the granulated sugar and light brown sugar for 3 to 4 minutes until the mixture becomes wonderfully light and fluffy. Remember to scrape down the sides of the bowl as you go.
Reduce your mixer speed to low. Add in the whole egg, then the extra egg yolk, mixing just until they're fully combined. Next, stir in the vanilla extract and almond extract, mixing until everything is smooth and incorporated.
With the mixer still on low, gradually add your dry ingredient mix to the wet ingredients, about half a cup at a time. Mix until you no longer see any white streaks of flour. Finally, gently fold in the chilled jimmies sprinkles until they are evenly distributed throughout your vibrant dough.
Line a baking sheet with parchment paper. Using a 2-tablespoon scoop, form generous dough balls and place them on the lined sheet. This chilling step is key for perfect cookies – refrigerate the dough balls for at least 2 hours, or even overnight.
When you're ready to bake, preheat your oven to 350°F (175°C) and line a fresh baking sheet with parchment paper. Pour the extra granulated sugar into a shallow bowl.
Take a few chilled dough balls from the fridge. Roll each ball gently in your hands, then roll it thoroughly in the granulated sugar until it's nicely coated. Place 5-6 sugared dough balls on your prepared baking sheet, ensuring they have plenty of room to spread. Keep the rest of the dough chilled.
Bake for 11-13 minutes, or until the edges are just slightly golden and the centers still look soft. Let these delightful cookies cool on the baking sheet for 5 minutes to set, then transfer them to a wire rack to cool completely. Enjoy your homemade, colorful treats!