Preheat oven to 350°F. Grease a 9x9 baking pan. You can also line the baking pan with foil, leaving a couple inches of overhang on two sides (this will allow you to easily lift the whole pan of bars out of the pan). Spray or grease the foil.
Using a hand mixer or stand mixer, beat butter and sugars on medium speed for several minutes, until light and fluffy. Add egg, egg yolk, vanilla, and sour cream and beat until combined, scraping the sides of the bowl as necessary.
In a separate bowl, combine flour, baking soda, salt, and 4 teaspoons chai spice.With the mixer on low, slowly add the the dry ingredients to the wet and mix until just combined (do not over-mix). Spread the dough into an even layer in the prepared pan (it will be thick). Bake for 20-23 minutes, until a toothpick inserted into the center comes out clean and the center no longer jiggles. Be careful not to over bake. Cool completely on a wire rack before frosting.
With a hand mixer or stand mixer, beat butter and powdered sugar for 1-2 minutes, until light and creamy. Add half and half, vanilla extract, and 2 teaspoons chai spice. Beat until completely combined, scraping the sides of the bowl as necessary. Spread the frosting on top of the cooled bars. If desired, combine the topping ingredients and sprinkle over the frosted bars.
Chai spice mix: 1 teaspoon cardamom, 1 teaspoon ground allspice, 3 teaspoons ground cinnamon, ½ teaspoon ground cloves, and 3 teaspoons ground ginger. Store leftovers in your spice cabinet.