In a large mixing bowl, beat together the unsalted butter and cream cheese until smooth and creamy. Gradually add granulated sugar, mixing until light and fluffy, about 2 minutes.

In a separate bowl, whisk together the all-purpose flour, cornstarch, fine salt, and baking powder. Gradually add this mixture to the wet ingredients, mixing on low speed until just combined.

Divide the cookie dough into two equal portions, shape into discs, wrap in plastic wrap, and chill for at least 2 hours or freeze for 30 minutes.

Roll out one disc of dough on a floured surface to about 1/3 inch thick. Cut out bunny shapes and place on a lined baking sheet. Chill again for 15 minutes.

Preheat the oven to 350°F (175°C). Bake the cookies for about 10 minutes or until edges are set but centers are soft. Transfer to a wire rack to cool completely.

Prepare the buttercream frosting by beating softened unsalted butter, adding powdered sugar gradually, mixing in salt and heavy cream until smooth. Add gel food coloring if desired.

Once the cookies are cooled, frost them with buttercream, dip in sanding sugar, and use piping bags to decorate with different designs.