For the custard:
Pour the milk into the top of a double boiler. Set it over a pan of simmering water over medium heat. Warm the milk for about 15 minutes or until it has scalded or reached a temperature of about 180-degrees.
While the milk is heating beat the eggs until they are light and thickened, about 3 minutes.
Add the sugar, the flour, the butter and the kosher salt. Beat until the mixture is smooth.
Pour the egg mixture into the scalded milk and continue cooking over medium heat, until the custard is thick and coats the back of a spoon. Stir frequently so the custard stays smooth while it’s cooking.
Spoon the custard into a heatproof dish and let it cool for about 10 minutes then stir in the vanilla.
Place a piece of food wrap directly on top of the custard then place another piece of wrap over the dish. Chill for about 2 hours. The food wrap placed directly on the custard prevents a film from forming on top as it cools.
Fold the whipped cream into the chilled custard.
For the berries:
Rinse and sort the blueberries and set aside on a tea towel to dry.
Rinse and core the strawberries. Slice the strawberries in half and cover with the sugar and Cointreau. Stir to mix together. Let them rest for about 30 to 45 minutes.
For the lemon syrup:
Pour the lemon juice and the water into a small saucepan. Add the sugar. Warm over medium heat, stirring until the sugar has dissolved, about 5 minutes.
For the whipped cream topping:
Beat the cream on medium speed just until the cream starts to thicken. Add the powdered sugar along with the mascarpone cheese, almond extract and Cointreau. Continue beating until the cream forms stiff peaks. Chill while you assemble the trifle.
To assemble the trifle:
Slice the pound cake into thin slices, about ¼” each or ½” if you want thicker slices of cake in the trifle. Dip a pastry brush into the lemon syrup and brush one side of the cake with the syrup.
Line the bottom of a trifle dish or large bowl with a single layer of the pound cake, cutting or overlapping to fit.
Top with a third of the custard, spreading to cover the cake.
Spread a layer of blueberries over the custard. Top with another layer of the sliced pound cake.
Spoon another third of the custard on top of the cake, spreading to cover.
Line the edge of the dish with a row of strawberries and spread a layer of strawberries over the top of the custard.
Add another layer of pound cake, followed by the remaining custard. Top with blueberries.
Place another layer of pound cake on the blueberries, line the edge of the dish with another row of strawberries and spread remaining strawberries across the top of the cake.
Spread the whipped cream over the top of the strawberries. Cover and refrigerate until well chilled, about 4 to 6 hours or overnight.
Garnish with a handful of blueberries and dust with powdered sugar before serving.