In a mixing bowl, dissolve cornstarch in 1/4 of the milk. Combine the remaining milk with the sugar in the saucepan; bring to a boil and remove from heat.
Beat the yolks into cornstarch mixture. Pour 1/3 of the boiling milk into the egg mixture, whisking constantly.
Return the remaining milk to a boil. Pour into hot egg mixture in a stream, whisking until the cream thickens and comes to a boil. Remove from the heat and beat in the butter and vanilla and lastly, the praline paste.
Pour the cream into a stainless steel pan. Press plastic wrap directly against the surface, so it won't form a film. Chill immediately.
To Assemble Paris Brest: Split the pastry shell in half horizontally and remove any wet pieces of dough. Then pipe the whipped cream into the hollow centre of the shell.
Place the top half of the pastry shell on the whipped cream and dust with powdered icing sugar.