Hi I am trying to improve my cakes, icings & techniques & I am wondering what buttercream can I use that Ive seen in fillings after the cake is cut, its is so thick inside the cake & it looks like its almost an inch thick between the layers. Thanks

Sounds like a truffle filling.
I melt a block of almond bark (or 2 bags chocolate chips - or white chocolate) and quickly beat in 2 (8oz) cream chz at room temp. Beat til light and creamy. It will be thick when it cools. For more sweetness, add a cup of powdered sugar and vanilla (or any flavoring).