In a mixer bowl or other large bowl, combine the cake flour, sugar, baking powder, and salt. Using the paddle attachment, mix on low speed for 30 seconds to blend. Add half the chocolate paste and the butter and mix until the dry ingredients are moistened. Increase the speed to medium if using a stand mixer, or high if using a hand-held mixer, and beat for 1 minute to aerate and develop the structure. Scrape down the sides of the bowl. Gradually add the remaining chocolate paste in two batches, beating for 20 seconds after each addition to incorporate the ingredients and strengthen the structure. Scrape down the bowl. With a rubber spatula, fold in the optional chocolate mini chips or bittersweet chocolate.