Preheat oven to 350°F (180°C). Grease and flour a 9-inch (22 cm) cake pan, then line the bottom with parchment paper.
In a large bowl, whisk together the oat flour, baking powder, baking soda, cinnamon, salt, and ground walnuts.
In a separate bowl, whisk together the eggs and granulated sugar until light and fluffy. Add the melted butter, milk, and vanilla sugar, and whisk to combine.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool completely in the pan before inverting it onto a wire rack. Once cooled, slice the cake in half horizontally. Spread the bottom layer with the apricot jam mixed with rum.
Mix the punch extract with the remaining apricot jam and spread it on the second layer of the cake.
Make the chocolate cream: Heat the heavy cream in a saucepan until just simmering. Pour the hot cream over the finely chopped dark chocolate. Let it sit for a minute, then stir until smooth and glossy.
Frost the cake with the chocolate cream, spreading it evenly over the top and sides.
Refrigerate for at least 1 hour before slicing and serving the Oatmeal Punch Cake!
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