To Make the Custard

1. Put the egg yolks in a medium sized bowl and gently beat them together. Set aside.
2. Add the sugar, cornstarch and milk to a large saucepan and mix until smooth.
3. Cook, stirring continuously, over medium-high heat until mixture begins to thicken and bubble.
4. Reduce heat to medium and simmer for 2 minutes. Remove from heat.
5.  Temper the egg yolks by adding a little bit of the hot milk mixture to the yolks while whisking briskly. Once incorporated, add the tempered egg yolk mixture back into the saucepan with the rest of the milk mixture.
6. Place the pan back on the heat and bring to a light boil. Allow to boil for 2 minutes, stirring continuously.
7. Remove from heat and add the butter and vanilla extract. Stir until smooth.
8. Then pour custard into a bowl and cover with clear plastic wrap pressed against the top so that it doesn’t form a thick film. Set aside to cool to room temperature.

To Make the Whipped Cream and Assemble the Trifles

1. Add the heavy whipping cream, powdered sugar and vanilla extract to a large mixer bowl and whip on high speed until stiff peaks form. 
2. Fill piping bag fitted with a large round tip with whipped cream and set aside.
3. Place 4 mini jars (approximately 8-9 oz jars) on the counter and pour the milk into a small bowl. Wash, hull and chop the strawberries. Wash the blueberries and pat dry.
4. To build the trifles, break a shortbread finger cookie into 4 pieces, dip them into the milk for 2-3 seconds and place the pieces into the bottom of a jar. Repeat until all 4 mini jars have a bottom layer of dipped shortbread.
5. Top the shortbread in each jar with a layer of custard.
6. Add a layer of blueberries and strawberries on top of the custard.
7. Next, pipe a layer of whipped cream.
8. Repeat another layer of milk-dipped shortbread, followed by custard and berries, and a big swirl of whipped cream.
9. If desired, garnish with a piece of shortbread, a couple berries and some shortbread crumbs.
10. Refrigerate trifles for 3-4 hours before serving, so the cookies have a chance to soften up. Serve cold and consume within 3-4 days.