Mix the mascarpone, sugar and vanilla in a bowl with a rubber spatula until smooth. Fold in the pecans; cover and refrigerate until ready to serve.

Heat a large skillet or griddle over medium heat. Spread some butter on both sides of each cake slice (about 2 tablespoons total). Toast in the skillet until golden, about 2 minutes per side. Let cool.

Combine the brown sugar and the remaining 2 1/2 tablespoons butter in a skillet over medium heat. Cook, stirring, until melted and bubbly, about 3 minutes. Carefully add the cream and cook, stirring, about 1 more minute. Stir in the blackberries, smashing a few berries with the back of a spoon. Remove from the heat, then add the cognac and return to medium heat. Carefully tilt the pan so the cognac ignites (if using an electric stove, hold a lit match near the sauce to ignite it); cook until the flames subside.

Spread some mascarpone filling on 4 pound cake slices; close with the remaining 4 cake slices. Top with the blackberries and sauce.

Photograph by Andrew Mccaul