Cut the butter into cubes and leave it at room temperature for about 10 minutes to soften.
After 10 minutes, preheat the oven to 180°C/350°F (160°C/320°F for a ventilated oven).
Place the softened butter in a large bowl, add the sugar, and mix with an electric whisk until you achieve a smooth mixture.
Add the eggs and mix again until well combined.
Grate the zest of the lemon using a fine grater. Squeeze the juice from the remaining lemon.
Add the lemon zest, lemon juice, sifted flour, and baking powder to the mixture. Mix everything well with the whisk until just combined.
Place half of the batter into muffin tins, filling them about 1/3 full. Add a teaspoon of lemon jam to the center of each tin and cover with the remaining batter.
Bake in the preheated oven for 20 minutes (or 15-18 minutes in a ventilated oven).
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