In a medium bowl, whisk together flour, Hojicha powder, baking powder and salt. Set aside.
In a large mixing bowl with an electric mixer fitted with a paddle attachment, beat together butter and sugar on medium speed until light and fluffy, about 3 minutes. Add the egg and vanilla, beat for 1 minute. With the mixer off, add the flour mixture all at once. Mix on low speed until the dough comes together and the flour has disappeared. Divide the dough into half.
Place half the dough in between 2 parchment papers. Using a rolling pin, roll into ¼-thick. Transfer to a baking sheet (with the parchment papers). Repeat with the rest and place on top of the other sheet of dough. Chill in the fridge for 1 hour.
Preheat the oven to 350˚F (177˚C). Line 2 baking sheets with parchment papers.
Once the dough is chilled, remove 1 sheet from the fridge. Peel both parchment papers off. Place one of the parchment papers on a clean working surface and put the dough on top. Lightly dust a 1½-inch round fluted cookie cutter with flour, then cut out as many cookies as you can. Line them on the prepared baking sheet and space the dough apart for 1 inch. Transfer the baking sheet to the fridge and chill for 15 minutes.
Meanwhile, collect the leftover scraps and keep in the fridge.
Bake the cookies for 10 – 12 minutes. Bake until the cookies are light golden brown on edges and bottom. Transfer the baking sheet to counter and let the cookies rest on the sheets for 2 minutes. Transfer the cookies to a cooling rack to cool completely.
Repeat with the remaining dough. Gather the scarps and combine with the scraps in the fridge. Repeat until you have 48 cookie dough.
For the white chocolate topping, prepare a double boiler. In a medium pot, add enough water to reach 1-inch deep. Bring to a simmer over medium heat. Reduce heat to low. Add chocolate chips in a medium mixing bowl. Set it over the pot. Stir to melt the chocolate until just smooth. Remove from heat. Transfer half a pipping bag (or just use a fork or spoon), drizzle over the cookies. Return any leftover white chocolate to the original white chocolate mixing bowl. Set the bowl over the warm pot. Whisk in Hojicha powder until blended. Transfer to a new pipping bag, drizzle over the cookies, perpendicular to the white chocolate drizzle. Let the chocolate set for at least 1 hour at room temperature.