How to Make It
Preheat oven to 350°F. Line 2 baking sheets with parchment paper. In a large bowl, whisk together the flour, baking powder, and salt. In another large bowl, beat the butter and sugar with an electric mixer on medium-high speed until light and fluffy, 2 to 3 minutes. Beat in the eggs, milk, and vanilla. Reduce the speed to low and gradually add the flour mixture, mixing just until incorporated.
Drop 2-inch mounds of dough (about 3 tablespoons each) two inches apart on the prepared baking sheets, about six per pan. Bake, rotating the sheets halfway through, until cookies are just set, 14-16 minutes. Slide parchment off hot sheet pans onto counter. Let cookies cool completely on parchment. Repeat with remaining batter, cooling baking sheet with cold water in-between batches.
Make the icing: In a large heatproof bowl, combine the powdered sugar and ½ cup boiling water. Whisk until the mixture is smooth and spreadable. Set bowl over a saucepan of gently simmering water. Add the bittersweet chocolate and corn syrup and stir until the chocolate has melted.
To finish, coat the underside of each cookie (it's flatter) with chocolate frosting. Let frosting set, about 10 minutes.
Place the melted and cooled white chocolate in a piping bag fitted with a small round tip. Pipe four lines across each cookie, intersecting in the center. Create spiderwebs by piping concentric rings connected to the diagonal lines. Let stand at room temperature until white chocolate sets, about 30 minutes. Store in an airtight container at room temperature up to 2 weeks.