Pour hot melted butter over the chocolate in a pyrex bowl and stir until the mixture is completely smooth. You can also add room temperature butter and chocolate to a bowl and microwave in 30 second bursts until smooth.
Whisk the eggs and sugars together until the mixture is thoroughly mixed together and becomes light.
Add the (cooled) chocolate/butter mix to the eggs and sugar and stir to make a glossy batter.
Use a sieve to sift in the flour and cocoa powder. Add the nuts and pour into two 23cm/9inch square parchment lined baking tins. Bake for 20 minutes (or until the edges look cooked but the middle has a slight wobble).
When cooled completely, whisk together the cream cheese, mascarpone, sugar and vanilla. Spread this layer between the two cooled brownies and leave to set in the fridge.
Recipe by Cooking TV Recipes at https://cookingtv-channel.com/cheesecake-brownie-sandwiches/