Commercial butter has about 80% fat, 15% water and 5% solids. Depending on where you are located double cream has 48% fat (UK), 40% (Canada) and the remaining fraction is obviously water and little solids.
Supposed, you need 100g butter in your receipe. This means that the dough will have 80g fat and 15g water (for a simple calculation I omit the content of the milk solids). If you want to substitute butter with double cream, you will need about 165g double cream to have the same fat content in the dough as if you used butter. The problem is the water that you will have in the dough: You will have 83g water in the dough which is 5,5 times more. Since there is very likely little water in a biscuit receipe and it is not possible to adjust the amount of other liquids (because there are no other liquids) I doubt that this substitution will work with a biscuit receipe. With a cake receipe you might have more luck (e.g. leave 83g - 15g = 68g milk out).
Making a simple buttercream (as known as American buttercream) with double cream is not possible because it has almost no liquid. You could try other fillings like custard-based buttercreams. I am certain that there are plenty receipes for cake fillings with double cream.