Preheat the oven with top heat at 160°C and bottom heat at 180°C.

Sift the low-protein wheat flour and milk powder together.

Cream the butter, margarine, and icing sugar until pale and creamy.

Add sweetened condensed milk, rum, and vanilla extract to the creamed mixture.

Beat the butter mixture until light and expanded.

Add the egg yolk gradually to the butter mixture.

Beat until the egg yolk is fully incorporated.

Fold the sifted flour mixture into the batter.

Beat the egg white, salt, granulated sugar, and lime juice to glossy soft peaks.

Fold one-third of the whipped egg white into the yolk batter.

Fold in the remaining whipped egg white without overmixing.

Melt the dark chocolate and butter for brownies over a double boiler.

Whisk the whole egg and granulated sugar for brownies until the sugar dissolves.

Stir the melted chocolate mixture into the egg mixture.

Fold the low-protein wheat flour for brownies into the chocolate mixture.

Weigh 400 g of butter-cake batter into a separate bowl.

Mix cocoa powder and chocolate paste into the 400 g batter portion.

Weigh 200 g of butter-cake batter into another separate bowl.

Mix mocha paste into the 200 g batter portion.

Spoon the plain, chocolate, and mocha batters alternately into the tube pan.

Pour the brownie batter in the center and over the top of the cake batter.

Bake until cooked through, about 45 to 55 minutes.

Remove the cake from the pan and cool it completely on a cooling rack.