In a small saucepan, combine 250 g raspberries, 75 g sugar, and 1 tbsp water. Cook over medium heat, stirring occasionally, until the raspberries break down and the sugar dissolves, about 6–8 minutes.

Remove from heat and stir in 1 tsp lemon juice. Press the mixture through a fine mesh sieve for a smooth sauce, discarding seeds, until you have roughly 1/2 cup raspberry sauce. Chill until cool.

In a large mixing bowl, beat 500 g unsalted softened butter on medium-high speed until pale and fluffy, about 3–4 minutes.

Gradually add the 395 g sweetened condensed milk, beating on low first to combine, then medium-high until the mixture is smooth and spreadable.

Slowly add ½ cup chilled raspberry sauce, tasting as you go and adding more if desired for a stronger flavor.

For a brighter pink color, add a drop or two of pink food color and mix until evenly colored.

If the buttercream becomes too soft, chill briefly for 10–15 minutes in the refrigerator, then re-whip to regain a silky texture.