Large cakes are usually filled with cream by simply spreading the filling between layers, but mini cakes can be injected with cream filling using the same method you would use to would add filling to a cupcake or doughnut. This method works best with 4- to 6-inch round cakes that are no more than 4 inches tall. Try filling the mini cake with a custard-like pastry cream filling or lighter whipped cream. Whipped cream quickly returns to a liquid state, which could make the inside of the cake soggy, so you must incorporate gelatin or piping gel to stabilize the whipped structure.
Set the mini cakes on a cooling rack after baking and allow them to cool for about two to three hours until they are completely cool in the center.
Whip heavy whipping cream, powdered sugar, vanilla and piping gel with an electric mixer until firm peaks form to make stabilized whipping cream. To make pastry cream, cook half and half, flour, sugar, egg yolks, a dash of salt and vanilla over medium heat until thickened. Chill the cream filling in the refrigerator for a few hours before filling the cakes.
Insert a large round decorating tip in a decorating bag. Fill the bag with the chilled cream.
Push the decorating tip down through the top of the cake until you reach the center. If you have trouble pushing the decorating tip through the cake, cut a small circle of cake out of the center with a thin knife. Set the piece of cake aside.
Squeeze the cream out of the bag and into the cake. Place one hand on the cake so you can feel the pressure build up inside as you fill the cake with cream. Stop filling the cake when the filling no longer comes out easily or when the cake feels full. Too much filling can make the cake burst open, but the total amount of filling a cake can hold depends on its density. A fluffy mini cake with lots of air pockets might be able to hold up to 1/4 cup of cream filling, for example, while a dense mini pound cake is unlikely to hold much at all. Remove the decorating tip from the cake.
Set the circular piece of cake back into the hole you cut in the center, if applicable. The piece might not fit perfectly because of the filling, so trim the length as needed so it fits flush. Decorate the cake as desired.