Sugar cookie meets cheesecake in this brilliant holiday mash-up: a creamy vanilla cheesecake gets the royal icing treatment — no cookie platter required.
Special equipment: two pastry bags fitted with small round tips
Position an oven rack in the middle of the oven and preheat to 350ºF. Press the cookie dough into the bottom of a 9-inch springform pan (use the bottom of a measuring cup to help spread it out and make it even). Bake until the cookie is set and golden brown, 12 to 15 minutes. Let cool completely, about 30 minutes. Wrap the bottom and sides of the pan with a large piece of foil and put it in a large roasting pan. Reduce the oven temperature to 325ºF.
Beat the cream cheese and granulated sugar together in a large bowl with an electric mixer on medium speed for 1 minute. Add the sour cream and vanilla and beat until just combined. Mix in the eggs by hand, one at a time, until just combined. (Take care not to over-mix or the cheesecake will puff up and crack). Pour the batter into the springform pan. Add enough hot water to the roasting pan to come about halfway up the side of the springform pan.
Bake until the outside of the cake is set but the centre is still slightly loose, about 1 hour and 20 minutes. Turn the oven off and leave the cheesecake in the oven for another hour. Remove the cheesecake from the roasting pan to a cooling rack. Run a knife around the edge and cool to room temperature. Cover and refrigerate at least 8 hours.
Run a knife around the edge of the pan once more and unlock to unmold the cheesecake. Transfer to a serving platter or cake stand and smooth the edges with a knife. Pat the Jimmie sprinkles into the sides of the cheesecake making sure not to cover the crust or get any on top.
Whisk together the confectioners’ sugar and meringue powder in a large bowl. Add 5 tablespoons water and beat with an electric mixer on medium-high speed until soft glossy peaks form, adding up to 1 more tablespoon water if necessary.
Transfer about ½ cup of the royal icing to a pastry bag fitted with a small round tip. Thin the remaining icing with 1 teaspoon of water at a time until it is the consistency of syrup (you know you have reached the right consistency if you drizzle a bit of the icing and it sinks back into the icing within 8 to 10 seconds). Transfer 1 cup of the thinned icing in another pastry bag fitted with a small round tip. Divide the remaining batter between two small bowls and colour one red and one green with the red and green gel food colouring.
Use the thick white icing to pipe a large scalloped circle around the cake leaving a ¾-inch border. Use the thinned white icing to fill the centre of the scalloped circle. Use an offset spatula to spread the icing out so that it is smooth. Decorate with the coloured icing while the white icing is still set. Chill the cake until the icing is set, at least 1 hour.