Sugar-Free Banana Cream for Cakes

Ingredients:

• 50g butter 
• 5 bananas or plantains (500g) 
• 1/2 lemon (30ml) 
• 300ml milk (hot or cold) 
• 50g cornstarch (can be replaced by 100g flour) 
• 3 egg yolks 
• 1 teaspoon vanilla essence 
• 500ml very cold cream (milk cream or vegetable cream)