Recipe(tried): Assembling a cake with filling the day before

Split each layer in half lengthwise. Lift the top one away.

Using your favourite buttercream recipe, smear a thin layer across the two cut layers.

Fill an icing bag with buttercream.

To keep the filling from seeping out, pipe the outer edge to make a "wall" to keep filling in place.

Fill the layer - not too much.

After filling the bottom slice, gently replace the top onto the bottom layer.

Proceed with other cakes or layers until complete.