1. Make the bonbons: Preheat the oven to 325 degrees. Line a baking sheet with a Silpat baking mat or parchment paper, and set aside.

2. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and 1/2 cup confectioners' sugar on medium speed. Add 1/4 teaspoon lemon extract, lemon zest, and salt. Mix to combine.

3. Add milk, and reduce mixer speed to low. Slowly add cake flour, and mix until well blended and the dough pulls away from the sides of the bowl.

4. Transfer the dough to a clean work surface, and divide in half. Roll each half into a 14-inch-long log. Cut each log into fourteen 1-inch pieces. Roll each piece into a dome. Place domes on prepared baking sheet, spaced 1 1/2 inches apart. If dough becomes too soft to handle, chill for a few minutes.

5. Bake bonbons until firm but not golden, 25 to 27 minutes. Transfer bonbons to a wire rack set over a piece of waxed paper to cool.

6. Make the glaze: In a small bowl, stir together remaining 3 cups confectioners' sugar, 1/4 cup hot water, corn syrup, oil, and remaining 1/2 teaspoon lemon extract until smooth. Add cassis, and stir until smooth.

7. Pour half of the warm glaze over the tops of the bonbons. Let bonbons set, about 15 minutes.

8. Repeat Step 7. Wait until glaze is dry, then decorate bonbons with royal icing: Place royal icing in a pastry bag fitted wtih the tip of your choice; pipe icing onto bonbons. Place decorated bonbons on a wire rack to set.