In a small saucepan, combine the thawed raspberries, 75 g sugar, and 1 tbsp water over medium heat.

Stir gently as the berries soften and release juice, about 5–7 minutes.

Add 1 tsp lemon juice, simmer for another 2 minutes to brighten the flavor.

Remove from heat and press the mixture through a fine-mesh sieve into a bowl, discarding seeds. Let cool completely.

Measure out ½ cup of the strained raspberry sauce for the buttercream. Reserve any extra for drizzling or a more intense flavor.

In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), beat 500 g softened unsalted butter on medium speed until pale and fluffy, about 3–5 minutes.

With the mixer on low, slowly pour in the 395 g sweetened condensed milk in a steady stream, then increase speed to medium-high and beat until fully incorporated and glossy, 2–3 minutes.

Add the ½ cup cooled raspberry sauce and beat on medium until the color and flavor are even, about 1–2 minutes.

If desired, add a drop or two of pink or red food color until you reach your preferred shade; mix briefly to combine.

Taste and adjust: if you want a stronger raspberry note, fold in a tablespoon more of the reserved sauce (but be careful — too much liquid can soften the buttercream).

Use immediately to frost, or chill slightly to firm before piping.