Make crêpes: Pour milk, flour, eggs, oil, sugar, vanilla, and salt into a blender. Blend, starting on low speed and finishing on high, until combined. Refrigerate batter for at least 30 minutes.

Make fruit mixture: Scoop strawberry jam into a small saucepan. Rinse out the jar with water and pour into the saucepan. Bring to a simmer over medium heat and stir. Simmer for 1 minute, remove from heat, and stir. Let cool to room temperature.

Brush some butter over a nonstick skillet over medium heat. Pour in 1/4 cup batter; tilt the skillet to coat evenly. Cook until crêpe bubbles and browns, about 1 minute and 30 seconds per side. Transfer to a baking sheet. Repeat with remaining batter, buttering the skillet between crêpes. Let crêpes cool completely before stacking, at least 15 minutes.

Make filling: Beat together cream, mascarpone cheese, sugar, and vanilla in a medium bowl with an electric mixer until stiff peaks form.

Lay a crêpe onto a large plate. Spoon on 2 to 3 tablespoons filling, spreading it almost to the edge. Swirl in some fruit mixture. Transfer finished crêpe onto a flat serving plate. Repeat with remaining crêpes, filling, and fruit mixture. Center each crêpe over the previous one in a stack. Gently press on a final plain crêpe to finish.

Refrigerate until completely chilled before cutting and serving.