Preheat the oven to 350°F. Line an 8 X 8-inch baking pan with parchment paper, with enough paper to overhang the sides of the pan. Set aside.
Whisk together the all-purpose flour and the baking powder. Set aside.
Using an electric mixer (hand or stand mixer), cream together ½ cup butter and the granulated sugar until light and fluffy, about 2 to 3 minutes. Add the egg and 1 teaspoon Rodelle Vanilla Paste. Mix until combined.
Add the flour mixture and mix on low speed until the dough comes together. Add 1 tablespoon of milk and mix until combined.
Scrape the cookie dough into the prepared pan. Using damp, clean fingertips, evenly press the dough into the pan. Use an offset spatula to smooth the dough on top, if desired.
Bake the cookie bars for 20 minutes, or until the edges start to turn golden. Remove from the oven and allow to cool on a wire rack for 10 minutes.
After 10 minutes, use the parchment paper to lift the bars out of the pan. Continue to cool the bars on a wire rack before frosting.
Meanwhile, as the cookie bars cool, make the frosting. Using an electric mixer (hand or stand mixer), beat the remaining ¾ cup butter until smooth. Stop the mixer and add the powdered sugar. With the mixer on low speed, mix until the butter and sugar begin to come together.
Turn the mixer up to medium and mix until smooth. Add the remaining 1 teaspoon Rodelle Vanilla Paste and 1 tablespoon milk. Mix on medium-high speed until light, fluffy, and very pale in color, about 2 to 3 minutes. If the frosting is too thick to spread, add in a little bit more of the milk until smooth and spreadable.
To create the watercolor frosting effect, tint the frosting with food coloring. Using an offset spatula, spread the cooled cookie bars with different colors of frosting. Create swirls in the frosting to combine the colors and top with sprinkles, if derided. Cut into bars and serve.
Cookie bars may be stored in an airtight container at room temperature for up to 3 days.
