Place thawed blueberries, sugar and lemon juice in a medium saucepan and stir to mix.

Heat mixture over medium-low heat until the mixture starts to lightly simmer. Whisk the mixture constantly so it doesn’t burn.

In a small bowl, stir the cornstarch with the water until the cornstarch is fully dissolved.

Whisk the cornstarch mixture into the blueberry mixture. Whisk this together until well blended. Continue to lightly simmer in the saucepan, whisking constantly, for 1 minute. This allows the mixture time to thicken.

Remove the saucepan from the heat. Allow blueberry cake filling to cool to room temperature before moving to next step.

Place room temperature blueberry cake filling into a medium bowl and cover with food-safe plastic wrap. Press the food-safe plastic wrap down against the top of the cake filling. Doing this helps to prevent the cake filling from developing a thicker top layer.

Place the blueberry cake filling into the fridge for at least 12 hours or overnight. This is important so that the cake filling has a chance to thicken before using in your cake.

Use the blueberry cake filling in a 3-layer cake (3 layers of cake with two layers of filling). If the filling still has too much liquid for your liking, you can strain a little bit of the excess liquid (do not strain all of the liquid out though).

When placing the blueberry cake filling on top of one of the cake layers, start with a ring of frosting around the outer edge of the cake layer. This helps to keep the cake filling in place so it doesn’t drip out the sides of the cake and get mixed into your frosting.

Decorate cake as desired.

Refrigerate cake for at least an hour before serving.

The cake and filling should be kept in the refrigerator. The cake can be stored in the refrigerator (wrapped well in food-safe plastic wrap) for up to 3 days.

Leftover blueberry cake filling can be kept in a sealed food safe container in the fridge for up to a week.