Why do my cupcakes go sticky on the top when cooled? Also why don't my chocolate cupcakes rise but the plain ones do? Please help!
Usually if cupcakes go sticky on top once they are cooled it is either because they were stored in an airtight container before they were completely cold, causing some condensation to form, or they are being baked and cooled in a humid environment. Cake batters contain quite a lot of sugar and sugar tends to attract moisture (it is "hygroscopic"). So when the cupcakes stand in a humid environment (either humid weather or a slightly steamy kitchen) they can attract some moisture and the tops can turn a little sticky. Usually you can ice/frost the cupcakes without a problem, or you can trim the sticky top off with a small serrated knife before icing. Also make sure that the cupcakes are fully baked and have dry tops before you remove them from the oven.
Nigella notes in the Kitchen Confidential section of her book Kitchen that if she is baking with cocoa then the mixture may need an extra boost. She suggests adding 1/4 teaspoon bicarbonate of soda (baking soda) to each 150g (1 cup) flour, along with the baking powder required. Don't be tempted to add too much bicarbonate of soda though, as the cupcakes could rise up too much in the oven, then sink back on cooling.