In a medium saucepan, stir together the custard powder and 1/4 cup (60 mL) sugar. Slowly add milk, whisking to prevent lumps. Bring to a boil over medium heat, stirring frequently.
Transfer the custard to a heatproof bowl and set it in a larger bowl of ice, stirring occasionally until completely cooled, about 10 min. Mix in the yogourt and vanilla.
To assemble the trifle, spread 1/2 cup (125 mL) of the custard in the bottom of a 3 to 4 L glass bowl. Stand some of the sliced strawberries in a ring against the sides of the bowl. Add half of the cake cubes. Drizzle 1 tbsp (15 mL) liqueur, if using, over the cake. Add half the berries and half the remaining custard. Repeat layers. Recipe can be made ahead up to this point. Cover and chill in the refrigerator overnight.
To finish, lightly toast almonds in a skillet over medium-low heat until golden. Cool to room temperature. Using an electric mixer, whip the cream to stiff peaks, gradually beating in remaining sugar. Spread the whipped cream on top of the trifle, sprinkle with toasted almonds and serve.